add lxc-unshare man page, and make 'lxc-unshare -h' work.

Also fix some tabs-as-spaces in lxc_unshare.c itself.
lxc-unshare: run usage() on '-h'

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>