GDBserver: clean up 'cont_thread' handling

GDBserver: clean up 'cont_thread' handling

As no place in the backends check cont_thread anymore, we can stop
setting and clearing it in places that resume the target and wait for
events.  Instead simply clear it whenever a new GDB connects.

gdb/gdbserver/
2014-11-12  Pedro Alves  <palves@redhat.com>

	* server.c (cont_thread): Update comment.
	(start_inferior, attach_inferior): No longer clear cont_thread.
	(handle_v_cont): No longer set cont_thread.
	(captured_main): Clear cont_thread each time a GDB connects.