auth/credentials: handle situations without a configured (default) realm

auth/credentials: handle situations without a configured (default) realm

We should not have cli_credentials_get_realm() return "" without a
configured (default) realm in smb.conf.
Note that the existing tests with creds.get_realm() == lp.get("realm")
also work with "" as string.

At the same time we should never let cli_credentials_get_principal()
return "@REALM.EXAMPLE.COM" nor "username@".

If cli_credentials_parse_string() gets "OTHERDOMAIN\username"
we must not use cli_credentials_get_realm() to generate
a principal unless cli_credentials_get_domain() returns
also "OTHERDOMAIN". What we need to do is using
username@OTHERDOMAIN as principal, whild we still
use cli_credentials_get_realm to get a default kdc,
(which may route us to the correct kdc with WRONG_REALM
messages).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>