Create aurweb.spawn for spawing the test server

This program makes it easier for developers to spawn the PHP server
since it fetches automatically what it needs from the configuration
file, rather than having the user explicitly pass arguments to the php
executable.

When the setup gets more complicated as we introduce Python,
aurweb.spawn will keep providing the same interface, while under the
hood it is planned to support running multiple sub-processes.

Its Python interface provides an way for the test suite to spawn the
test server when it needs to perform HTTP requests to the test server.

The current implementation is somewhat weak as it doesn’t detect when a
child process dies, but this is not supposed to happen often, and it is
only meant for aurweb developers.

In the long term, aurweb.spawn will eventually become obsolete, and
replaced by Docker or Flask’s tools.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

conf/config.defaults

@@ -41,6 +41,9 @@ cache = none
 cache_pkginfo_ttl = 86400
 memcache_servers = 127.0.0.1:11211
 
+; Directory containing aurweb's PHP code, required by aurweb.spawn.
+;htmldir = /path/to/web/html
+
 [ratelimit]
 request_limit = 4000
 window_length = 86400