cli: fix_con() — do it better, some consistency work

* bugz/argparsers.py (make_parser): The default=None is not needed
now.
* bugz/cli.py (PrettyBugz): Rename password_cmd to passwordcmd,
rework fix_con() local function.
* bugz/configfile.py (Connection): Rename password_cmd to
passwordcmd.