Callback on successful mount for Arch Linux guests

Arch Linux will attempt to run `/etc/rc.d/vagrant-mounted` when a mount has been completed.

An environment variable, `MOUNTPOINT`, is passed to the job, the value of which is the path inside the guest that was mounted to.

The intention is that this script will trigger any other scripts that need to be run and exit immediately, rather than start a long-running daemon. This is to work around the fact that Arch Linux's `rc.d` system does not have support for events like Ubuntu's `upstart` does.