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the list received at startup or we get an out of date list.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)