browse: allow consecutive spaces in the header

Add a few super minor lint fixes

Other lint warnings left behind are ones due to external package and
due to maintainer's preferences.

Made a faulty assumption that virualhosts could share acme proxy handlers; turns out they can't without fumbling up the middleware configuration (middleware chains overlap and cross over into other virtualhosts)!

Hosts which are eligible for automatic HTTPS have default port "https" but other hosts (wildcards, loopback, etc.) have the default port 2015. The default host of empty string should be more IPv6-compatible.

proxy: make http header block scoped

proxy: make tests workable when offline

Instead of accessing the google website, we setup a local server
for test, then tests will work fine even we are offline.

Fix issue #346
Signed-off-by: Tw <tw19881113@gmail.com>

Each proxy block should could specify its own http header
instead of sharing a global one.

Fix issue #341
Signed-off-by: Tw <tw19881113@gmail.com>

proxy: 'except' property to ignore subpaths

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.