userdb: Fix return value of groupdb_by_name()

userdb: Fix return value of groupdb_by_name()

The commit 7419291670 ("userdb: move UserDBMatch handling from userdbctl
into generic userdb code to allow it to be done server side")
unintentionally passes return value from group_record_match() as its
return value and thus diverges from other search functions that return 0
on success. Align that by returning 0 instead of 1, all existing callers
are invariant to this change.