regression in exim4 DSA-2154-1

[Stefan Fritsch <sf@sfritsch.de>]

Unfortunately, the latest update introduced a regression: Testing of 
user filters with -bf as normal user no longer works:

	$ /usr/sbin/exim4 -bf .forward
	exim: changing group failed: Operation not permitted
	$

See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=611572