auth:creds: Guess the username first via getpwuid(my_id)

auth:creds: Guess the username first via getpwuid(my_id)

If we have a container, we often don't have USER or LOGNAME set.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14883

Tested-by: Anoop C S <anoopcs@samba.org>
Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit c28be4067463e582e378df402f812e510883d606)

Autobuild-User(v4-15-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-15-test): Mon Nov 15 11:34:06 UTC 2021 on sn-devel-184