fastcgi: Implement resolve symlinks feature

[endelwar]

This PR will enable resolving real path on symlinked releases.
When using a symlink based directory schema PHP, through Caddy, will serve old symlinked directory instead of resolving the actual real path of new released code: due to the nature of PHP opcache the new release path is not read by PHP because it’s cached and symlink path doesn’t change form PHP prospective.
Adding the new resolve_root_symlink subdirective on php_fastcgi will enable resolution of real path, thus permitting Caddy to server the actual codebase instead of the old one.

See this forum post to further details.

Example Caddyfile:

http://example.test {
    root * /var/www/deploy/current
    php_fastcgi 127.0.0.1:9000 {
        resolve_root_symlink
    }
}

Where /var/www/deploy/current is a symlink to real directory

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

Thanks!

I think resolve_symlinks would be a better name for the option, for consistency and brevity. We typically use underscores between words in option names.

[mholt]

Thanks! This is a pretty straightforward change.