* lib/idcache.c: Restore most of the 2006-11-06 patch, so as to

* lib/idcache.c: Restore most of the 2006-11-06 patch, so as to
continue using the flexible array member (thus, this module performs
half as many malloc calls), with the addition that...
(getgroup, getuser): Consistently record a non-match via an empty
"name" string, and map an empty string match to a NULL return value.
* modules/idcache (Depends-on): Re-add flexmember.