errors: Set missing Content-Type for plaintext error messages

And corrected an error in a copy and pasted comment

It was implemented for almost a year but we'll probably never use it, especially since we'll match more than the path in the future.

Now attempt to staple OCSP even for certs that don't have an existing staple (issue #605). "tls off" short-circuits tls setup function. Now we call getEmail() when setting up an acme.Client that does renewals, rather than making a new account with empty email address. Check certificate expiry every 12 hours, and OCSP every hour.

Rotate process log

This fixes a regression introduced in recent commits that enabled TLS on the default ":2015" config. This fix is possible because On-Demand TLS is no longer implicit; it must be explicitly enabled by the user by setting a maximum number of certificates to issue.

Otherwise it tries to create an account and stuff at first start, even without a Caddyfile or when serving localhost.

If Caddy is running but not listening on port 80, reloading Caddy with a new Caddyfile that needs to obtain a TLS cert from the CA would fail, because it was just assumed that, if reloading, port 80 as already in use. That is not always the case, so we scan the servers to see if one of them is listening on port 80, and we configure the ACME client accordingly. Kind of a hack... but it works.

After 10 certificates are issued, no new certificate requests are allowed for 10 minutes after a successful issuance.

Biggest change is no longer using standard library's tls.Config.getCertificate function to get a certificate during TLS handshake. Implemented our own cache which can be changed dynamically at runtime, even during TLS handshakes. As such, restarts are no longer required after certificate renewals or OCSP updates.

We also allow loading multiple certificates and keys per host, even by specifying a directory (tls got a new 'load' command for that).

Renamed the letsencrypt package to https in a gradual effort to become more generic; and https is more fitting for what the package does now.

There are still some known bugs, e.g. reloading where a new certificate is required but port 80 isn't currently listening, will cause the challenge to fail. There's still plenty of cleanup to do and tests to write. It is especially confusing right now how we enable "on-demand" TLS during setup and keep track of that. But this change should basically work so far.

proxy: Support unix sockets

fastcgi: New function DialWithDialer to create FCGIClient with custom Dialer.

Making directives externally registerable

templates: Add .Markdown directive

This allows any template to use:
{{.Markdown "filename"}} which will convert the markdown contents
of filename to HTML and then include the HTML in the template.

markdown: enable definition lists

When the requested host is not found, log the remote host.

letsencrypt: properly retrieve hostname from request.

fastcgi: IPv6 when parsing r.RemoteAddr

basicauth: fixed 'go vet' printing function value