Add a master flag to struct server_entry

Add a master flag to struct server_entry

URI lookups will contain a flag to indicate if the KDC is a master or
normal KDC, so add a master variable to server_entry.  The default
value is -1 and means that the status has not been determined by the
information source.  If the flag is set, use it to avoid a seperate
lookup in k5_kdc_is_master().

Also add a "master" argument to add_host_to_list().  Adjust the
existing calls of add_host_to_list() for the new argument (set to -1
as they are from non-URI lookups).

ticket: 8496