parser: Allow use of environment variables in tokens

It no longer uses regular expressions.
It supports both the Unix `{$ENV_VAR}` _and_ the Windows `{%ENV_VAR%}`
syntax.
Added test for both Unix and Windows env. syntax.

Go programs using the caddy package may not want the it to capture all the signals...

Added a -grace flag to customize graceful shutdown period, fixed bugs related to closing file descriptors (and dup'ed fds), improved healthcheck signaling to parent, fixed a race condition with the graceful listener, etc. These improvements mainly provide better support for frequent reloading or unusual use cases of Start and Stop after a Restart (POSIX systems). This forum thread was valuable help in debugging: https://forum.golangbridge.org/t/bind-address-already-in-use-even-after-listener-closed/1510?u=matt

Fixed pidfile writing problem where a pidfile would be written even if child failed, also cleaned up restarts a bit and fixed a few bugs, it's more robust now in case of failures and with logging.

Whether the original parent process or a child process as part of a restart, the pidfile will not be written/changed until that process has started successfully. It is written every time caddy.Start() succeeds (may be reundant, but that's probably okay).

Added test for the fix of issue #304

fixed test failure on CPU 1 core machine.

gzip: added Vary: Accept-Encoding to response header.

When CPU is 1 core, expected value (int(0.5 * float32(maxCPU))) is zero.
But runtime.GOMAXPROCS(-1) returns always 1.

When the downstream is cache server or CDN, it is important.

startup: Only run commands at first startup

Actually, restart on posix systems failed entirely if caddy was executed without the path included; also fixed a related bug where a variable was declared but never assigned.

We had to hack some special support into the server and caddy packages for this. There are some middlewares which should only execute commands when the original parent process first starts up. For example, someone using the startup directive to start a backend service would not expect the command to be executed every time the config was reloaded or changed - only once when they first started the original caddy process.

This commit adds FirstStartup to the virtualhost config

Alphabetize command line options, add -pidfile

As per https://github.com/mholt/caddy/issues/331

As per https://github.com/mholt/caddy/issues/317

Some programs (Node.js, supervisor, etc.) open a stdin pipe by default and don't use it, causing Caddy to block. It is their error, but we have to try to accommodate unfortunately. To fix this more universally, parent must explicitly set -conf to "stdin" to read from pipe.

Since a directive can appear on multiple lines, the object syntax wasn't working well. This also fixes several other serialization bugs.

The error channel used when starting all the servers must be buffered so that, even if there are no errors at startup, the returns that insert into the error channel will not be blocked, since after startup, nobody is reading that channel anymore.

Wrote tests for browse.go and redir.go

The base path being optional in the Caddyfile is convenient when you just want the whole site to be markdown-enabled. The other change is to always generate links... this is because an index page for markdown files may not be statically generated, but it should still show links. Commit 09341fca was a regression, and this fixes it.

It can take a few seconds...