CLDAP: Fix domain handling in netlogon requests

1. Stop using getdomainname() as it is often not properly initialized
2. The code using getdomainname() was not working anyway it was trying to
look at the function call output in hostname which is always empty at that
point.
3. Always check the requested domain matches our own, we cannot reply to
anything else anyway.

Pre-requisite to fix: https://fedorahosted.org/freeipa/ticket/3639

Signed-off-by: Simo Sorce <simo@redhat.com>
CLDAP: Return empty reply on non-fatal errors

Windows DCs return an empty reply when a legal request cannot satisfied.
If we get EINVAL or ENOENT it means the information requested could not be
found or input parameters were bogus.
Always return an empty reply in these cases.

On any other internal error just return, the request may have been legit but we
can't really handle it right now, pretend we never saw it and hope the next
attempt will succeed.

Fixes: https://fedorahosted.org/freeipa/ticket/3639

Signed-off-by: Simo Sorce <simo@redhat.com>