Add support for new create=(dir,file) mount option

Just like we already had "optional", this adds two new LXC-specific
mount flags:
 - create=dir (will do a mkdir_p on the path)
 - create=file (will do a mkdir_p on the dirname + a fopen on the path)

This was motivated by some of the needed bind-mounts for the
unprivileged containers.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>