libads: Fix fallback logic when finding a domain controller

libads: Fix fallback logic when finding a domain controller

This is a patch to fix bug 11321.

When finding a domain controller, the method is to resolve
the IP address of candidate servers, and then do an ldap ping until a
suitable server answers.

In case of failure, there's fallback from DNS lookup to netbios lookup
(if netbios is enabled) and then back to site-less DNS lookup. The two
problems here are:
1. It makes more sense to try site-less DNS before NetBIOS because the
fallback to NetBIOS is not likely to give better results.
2. The NetBIOS fallback screws the site-less fallback (I suppose the
"goto considered harmful fellows are sometimes right after all...).

This fix extracts the core code that does name resolving+ldap ping
into a separate function and then activates this function in up to
three modes - site-aware, site-less, and netbios, in that order.

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>