s4-samba-tool: support help, and show description of commands

when you don't specify a subcommand, show the description of the
subcommands in the list of available subcommands. Also show the list
of subcommands when you use 'help', '--help' or '-h' as a subcommand

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Sun Nov 28 01:56:46 CET 2010 on sn-devel-104