Configuring winbindd to retrieve id mappings through idmap_ad does not
work in a clustered setup. log.winbindd-idmap shows:

[2016/09/27 20:55:51.624381,  2, pid=31988, effective(0, 0), real(0, 0)] ../lib/tdb_wrap/tdb_wrap.c:64(tdb_wrap_log)
  tdb(/var/lib/samba/private/secrets.tdb): tdb_open_ex: could not open file /var/lib/samba/private/secrets.tdb: No such file or directory
[2016/09/27 20:55:51.624440,  3, pid=31988, effective(0, 0), real(0, 0)] ../lib/dbwrap/dbwrap_tdb.c:430(db_open_tdb)
  Could not open tdb: No such file or directory
[2016/09/27 20:55:51.624474,  1, pid=31988, effective(0, 0), real(0, 0)] ../auth/credentials/credentials_secrets.c:399(cli_credentials_set_machine_account_db_ctx)
  Could not find machine account in secrets database: Failed to fetch machine account password from secrets.ldb: error and failed to open /var/lib/samba/private/secrets.tdb: NT_STATUS_CANT_ACCESS_DOMAIN_INFO
[2016/09/27 20:55:51.624507, 10, pid=31988, effective(0, 0), real(0, 0), class=winbind] ../source3/winbindd/idmap_ad.c:314(idmap_ad_get_tldap_ctx)
  idmap_ad_get_tldap_ctx: cli_credentials_set_machine_account failed: NT_STATUS_CANT_ACCESS_DOMAIN_INFO

The problem here is that idmap_ad only looks in a local tdb file for the
credential information, not a clustered tdb. A secondary problem is that
this error is incorrectly handled in the main thread.

Patches to follow.