Use with_unix_fd_list variant of DBus method call.

Any returned 'h' type argument is intended to be an index into a
unix fd list structure obtained separately. Such a unix fd list, in
python, will manage and close any contained fds as a whole, or fds may
be retrieved, managed, and closed individually, as is done in this
patch.

Also, use None instead of -1 because python.

This patch means gajim keeps its supsend inhibitor open and stops it
closing stdin on suspend.

Fixes #8852