fastcgi: Implement try_files override in Caddyfile directive

[francislavoie]

See #4344, this is part 2 of completing that issue. This should be merged after #4346.

This makes it possible to override the try_files logic in the php_fastcgi directive, without having to implement a try_files to run before php_fastcgi (plenty of examples of users doing this on the forums, where their PHP apps didn't quite conform to what they needed).

It makes the following config possible:

:8080 {
	root * /srv
	php_fastcgi localhost:9000 {
		try_files {path} {path}/index.php =404
	}
	file_server

	handle_errors {
		respond "Error!"
	}
}

[francislavoie]

@mholt with #4346 merged, this is ready to go

[mholt]

Great, thanks for the test.