libc/nss: Ensure that setgroupent(3) actually works as advertised

Because the "files" and "compat" implementations failed to set the
"stayopen", keyed lookups would close the database handle, contrary to
the purpose of setgroupent(3). setpassent(3)'s implementation does not
have this bug.

PR: 165527
Submitted by: Andrey Simonenko
MFC after: 1 month