Improve error message and backoff as well

# Conflicts:
#	plugins.go
#	vendor/manifest

* Updated lucas-clemente/quic-go for QUIC 39+ support

* Update quic-go to latest

* Trim path prefix using EscapedPath()

* clarify comments

* Added Tests for trimPathPrefix

* Ensure path with trailing slash is properly trimmed

* Updated tests to match prepatch behaviour

* Updated tests to match prepatch behaviour

* call parse on url rather than instance

* add additional tests

* return unmodified url if error.  Additional tests

tls: Restructure and improve certificate management

Only strip the port from the Location URL value if the port is NOT the
HTTPSPort (before, we compared against DefaultHTTPSPort instead of
HTTPSPort). The HTTPSPort can be changed, but is done so for port
forwarding, since in reality you can't 'change' the standard HTTPS port,
you can only forward it.

See discussion on #2015; the initial change had removed this check, and
I can't remember why I removed it or if it was accidental. Anyway, it's
back now.

Re: #2009, 1.9 or newer is needed because of the introduction of `sync.Map`

See discussion on #2015 for how this situation was discovered. For a
Caddyfile like this:

	localhost {
		...
	}
	:2015 {
		...
	}

Running Caddy like this:

	caddy -host localhost

Produces two sites both defined as `localhost:2015` because the flag
changes the default host value to be `localhost`. This should be an
error since the sites are not distinct and it is confusing. It can also
cause issues with TLS handshakes loading the wrong cert, as the linked
discussion shows.

# Conflicts:
#	sigtrap_posix.go

Also introduce caddy.OnProcessExit which is a list of functions that
run before exiting the process cleanly; these do not count as shutdown
callbacks, so they do not return errors and must execute quickly.

Fixes #1961

According to RFC 7231 and RFC 7230, there's
no reason a GET-Request can't have a body
(other than it possibly not being supported
by existing software). It's use is simply not
defined, and is left to the application.

Not a huge issue, but has security implications if OAuth tokens leaked

- Also implemented robust error handling and failovers
- Vendored klauspost/cpuid

- Expose the list of Caddy instances through caddy.Instances()

- Added arbitrary storage to caddy.Instance

- The cache of loaded certificates is no longer global; now scoped
  per-instance, meaning upon reload (like SIGUSR1) the old cert cache
  will be discarded entirely, whereas before, aggressively reloading
  config that added and removed lots of sites would cause unnecessary
  build-up in the cache over time.

- Key certificates in the cache by their SHA-256 hash instead of
  by their names. This means certificates will not be duplicated in
  memory (within each instance), making Caddy much more memory-efficient
  for large-scale deployments with thousands of sites sharing certs.

- Perform name-to-certificate lookups scoped per caddytls.Config instead
  of a single global lookup. This prevents certificates from stepping on
  each other when they overlap in their names.

- Do not allow TLS configurations keyed by the same hostname to be
  different; this now throws an error.

- Updated relevant tests, with a stark awareness that more tests are
  needed.

- Change the NewContext function signature to include an *Instance.

- Strongly recommend (basically require) use of caddytls.NewConfig()
  to create a new *caddytls.Config, to ensure pointers to the instance
  certificate cache are initialized properly.

- Update the TLS-SNI challenge solver (even though TLS-SNI is disabled
  currently on the CA side). Store temporary challenge cert in instance
  cache, but do so directly by the ACME challenge name, not the hash.
  Modified the getCertificate function to check the cache directly for
  a name match if one isn't found otherwise. This will allow any
  caddytls.Config to be able to help solve a TLS-SNI challenge, with one
  extra side-effect that might actually be kind of interesting (and
  useless): clients could send a certificate's hash as the SNI and
  Caddy would be able to serve that certificate for the handshake.

- Do not attempt to match a "default" (random) certificate when SNI
  is present but unrecognized; return no certificate so a TLS alert
  happens instead.

- Store an Instance in the list of instances even while the instance
  is still starting up (this allows access to the cert cache for
  performing renewals at startup, etc). Will be removed from list again
  if instance startup fails.

- Laid groundwork for ACMEv2 and Let's Encrypt wildcard support.

Server type plugins will need to be updated slightly to accommodate
minor adjustments to their API (like passing in an Instance). This
commit includes the changes for the HTTP server.

Certain Caddyfile configurations might error out with this change, if
they configured different TLS settings for the same hostname.

This change trades some complexity for other complexity, but ultimately
this new complexity is more correct and robust than earlier logic.

Fixes #1991
Fixes #1994
Fixes #1303

Adding the bash command for downloading the caddy.service file from the reposetory. Because it was easy to forget where you find it.

Signed-off-by: Tw <tw19881113@gmail.com>

* shutdown: allow graceful shutdown for SIGTERM on posix

The signal is already trapped; make it do the same thing as SIGQUIT to
be more inline with Unix/Linux shutdown expectations.

Fixes #1993

* Implement comment feedback ideas

* Turn on KeepAlive in QuicConfig of RoundTripper

* Update reverseproxy.go

Signed-off-by: Tw <tw19881113@gmail.com>

Signed-off-by: Tw <tw19881113@gmail.com>