Parse hostname/port for AAA server addresses

  * Add methods to create a TR_AAA_SERVER from a hostname:port string
    - also a version starting from a TR_NAME, which is a bit of a
      misuse of the TR_NAME
  * Update code to use the new methods instead
  * tr_aaa_server_new() no longer sets the hostname
  * tr_aaa_server_set_port() only uses default port when port == 0,
    otherwise allows any value
  * refactor tr_cfg_parse_one_aaa_server() to better use talloc
  * Raise error in tr_tids_req_handler() if AAA server allocation fails