Clear the supplementary groups list when daemonising

Clear the supplementary groups list when daemonising the process.

This fixes the following issue raised by rpmlint:

	cachefilesd.x86_64: E: missing-call-to-setgroups-before-setuid /usr/sbin/cachefilesd

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>