ldap: Fix autodiscovery for Active Directory

[greezybacon]

There are a couple problems with the current autodiscover protocol in the plugin:

• Net_DNS2 does not support auto-discovery of DNS servers on Windows platforms, and so cannot lookup the LDAP servers
• The configuration pre_save does not properly handle empty DNS server listing, and so offers incorrect negative feedback when leaving the LDAP servers box and DNS servers boxes blank
• Multi-site Active Directory deployments may have servers that are far away from the osTicket server. Windows rates all servers with the same weight and priority. Therefore, osTicket will likely select a poor server from the list to bind to and search

This patch adds support for automatically discovering the closest domain controller (LDAP server) by asynchronously connecting to all domain controllers advertised in DNS, sorted by priority and weight, in parallel and using the first server to respond.

The protocol is described by Microsoft at
https://technet.microsoft.com/en-us/library/cc978016.aspx

[greezybacon]

Actually, with 2ms loop timing, this is 120ms (which is probably a long enough delay)

[greezybacon]

unset($sockets[$i]);

[Neustradamus]

@greezybacon: What is the status of this feature/pr several years after?