s3:pylibsmb: Add .unlink() API to SMB Py bindings

s3:pylibsmb: Add .unlink() API to SMB Py bindings

Add a basic .unlink() API to the source3 bindings. This is based on the
source4 python bindings, but uses the source3 client library APIs.
(We use a helper function to do most of the work, because we will need
to reuse it in order to support the deltree API).

Update the source4 test to use the source3 API. We will gradually
convert it over, and then delete the source4 python bindings.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>