Test that the host header forwarding on proxy

Support for placeholders in fastcgi env vars.

This is inconsistent with the other directives, but import is a special
case and frankly the behavior of import shouldn't change depending
on the directory from which you run caddy. Breaking change but I think
it's for the better, and best to do it now before 1.0.

Also see
https://forum.caddyserver.com/t/caddy-0-9-beta-version-available-updated-beta-2/146/29?u=matt
which has another account of strange proxy behavior; I think this
resolves that.

Also a few little formatting changes and comments.

If we listen on 127.0.0.1:80 for `localhost` but :80 for everything else,
then a hostname in the hosts file that resolves to 127.0.0.1 will be
served on :80 (unless the bind directive is used) but the OS will use
the socket listening at 127.0.0.1:80, thus giving a "No such site" error
even though the site is there, but it's on the other listener at :80.

Two ways to fix this: 1) Leave as-is and require the user to set "bind
127.0.0.1" in their Caddyfile for all sites that are resolved in the
hosts file, or 2) Take out this special case and let localhost sites
listen on :80 (unless the user changes that with the bind directive, of
course). Having localhost bind to any interface is a little annoying
(unsettling?) but probably best in the long run.

https://forum.caddyserver.com/t/wildcard-virtual-domains-with-wildcard-roots/221/9?u=matt

Add filemanager directive

Fix go get url

Fixes #904

OnStartup and OnShutdown callbacks now run as part of restarts, too.
The startup and shutdown directives only run their commands NOT as part
of restarts, as before. Some middleware that use OnStartup may need to
switch to OnFirstStartup and implement OnFinalShutdown to do any cleanup
as needed.

Minor refactor for rewrite.

* rewrite: fix new case
- added new test case and solution
- fix test case on rewrite_test

* replacer: change to table test

Fix missed if_op refactor. Ensure with tests.

* checkpoint

* Added RequestMatcher interface. Extract 'if' condition into a RequestMatcher.

* Added tests for IfMatcher

* Minor refactors

* Refactors

* Use if_op

* conform with new 0.9 beta function changes.

Also fixed a few typos and renamed caddyfile.ServerBlocks() to
caddyfile.Parse().

Refactoring to remove lint warnings

The search-and-replace was a little too aggressive and I accidentally
ran tests recursively in a subdirectory instead of repo's top folder.

- Server types no longer need to store their own contexts; they are
  stored on the caddy.Instance, which means each context will be
  properly GC'ed when the instance is stopped. Server types should use
  type assertions to convert from caddy.Context to their concrete
  context type when they need to use it.
- Pass the entire context into httpserver.GetConfig instead of only the
  Key field.
- caddy.NewTestController now requires a server type string so it can
  create a controller with the proper concrete context associated with
  that server type.

Tests still need more attention so that we can test the proper creation
of startup functions, etc.

* Add timeout to http get on health_check

* Add new test and up the timeout

* Tests for change to default timeout

* Only call http client once and make options more inline with current caddy directives

Refactor and clean up policy code

This commit shouldn't change any behavior. It is simply a cleanup of
the different proxy policies. It also adds some comments explaining the
sampling method used, since on first inspection it might not appear to
be a uniformly random selection.