If enterprise principals are enabled (which is the default in the AD
provider), then the returned UPN might be slightly different from
the one SSSD constructs before attempting the login. This patch makes
SSSD only check if the principal is the same when the enterprise
principals are disabled.

Because we now always store SIDs in the LDAP provider, we also need to
always initialize the ID mapping context even if ID mapping itself is
off.

https://fedorahosted.org/sssd/ticket/1504

Implements dynamic DNS updates for the AD provider. By default, the
updates also update the reverse zone and run periodically every 24
hours.

This options is mostly provided for future expansion. Currently it is
undocumented and both IPA and AD dynamic DNS updates default to
GSS-TSIG. Allowed values are GSS-TSIG and none.

https://fedorahosted.org/sssd/ticket/1831

Adds a new option that can be used to force nsupdate to only use TCP to
communicate with the DNS server.

https://fedorahosted.org/sssd/ticket/1832

While some servers, such as FreeIPA allow the PTR record to be
synchronized when the forward record is updated, other servers,
including Active Directory, require that the PTR record is synchronized
manually.

This patch adds a new option, dyndns_update_ptr that automatically
generates appropriate DNS update message for updating the reverse zone.

This option is off by default in the IPA provider.

Also renames be_nsupdate_create_msg to be_nsupdate_create_fwd_msg

This is a requirement to update the PTR records.
Includes a unit test.

This new options adds the possibility of updating the DNS entries
periodically regardless if they have changed or not. This feature
will be useful mainly in AD environments where the Windows clients
periodically update their DNS records.