Prevent the -g option from preserving groups that a non-root receiver

Prevent the -g option from preserving groups that a non-root receiver
does not belong to, in these two ways:
    1. If a group mapping doesn't exist for a group name, do not preserve
	it for a non-root receiver.  This is especially evident with the
	sender is a daemon using chroot because then no mappings are
	available.
    2. Before setting the group on a file make sure that it is in the list
	of groups returned by getgroups().  The same thing is done by chgrp
	on systems that support bsd-style chown/chgrp, and this enforces
	that it happens the same way on all systems.  Overhead is very
	little, especially since most systems don't allow more then 16
	groups per user.