s3:pylibsmb: Add .deltree() API to SMB py bindings

s3:pylibsmb: Add .deltree() API to SMB py bindings

This basically re-uses the underlying functionality of existing APIs in
order to support a .deltree() API, i.e.
- we use the .list() functionality (i.e. do_listing()) to traverse every
  item in the given directory.
- we then use either .unlink() (i.e. unlink_file()) or .rmdir() (i.e.
  remove_dir()) to delete the individual item.
- sub-directories are handled recursively, by repeating the process.

Note that the .deltree() API is currently only really used for testing
(and deleting GPO files). So the recursion is never going to be
excessive.

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

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