Add state=empty_directory to file module

This allows to ensure that a directory exists and is empty. If it didn't
previously exist, it will be created, and if it did exist, children will
get recursively removed.

Note: I don't know how to mark this new *choice* to be added in version
1.3 in the docstring. Please tell me how…

Signed-off-by: martin f. krafft <madduck@madduck.net>