Always query the PDC for the list of trusted domains rather than interating

Always query the PDC for the list of trusted domains rather than interating
the list received at startup or we get an out of date list.  I thought
there might be some sequence number that is incremented when a trusted
domain is added or removed - perhaps there is but I just haven't found it
yet.

 - Renamed get_domain_info() to init_domain_list()

 - Made an accessor function to return the list of trusted domains rather
   than using a global so we don't have to remember to put a magic init
   function

 - The getent state can not keep a pointer to a winbind_domain structure as
   it may be freed if init_domain_list() is called again so we keep the
   domain name instead
(This used to be commit 37216c649a394b449eaaaa6644709eafb3bf37ff)