Implement remote multi-process extensions.

	Implement remote multi-process extensions.

	* remote.c (struct remote_state): Add extended and
	multi_process_aware fields.
	(remote_multi_process_p): New.
	(PACKET_vKill): New.
	(record_currthread): Use thread_change_ptid.  Notice new
	inferiors.
	(set_thread, remote_thread_alive): Use write_ptid.
	(write_ptid, read_ptid): New.
	(remote_current_thread, remote_threads_extra_info): Use them.
	(remote_threads_info): Likewise.  Detect new inferiors.
	(remote_start_remote): Add inferior to inferior list.
	(remote_multi_process_feature): New.
	(remote_protocol_features): Add "multiprocess" feature.
	(remote_query_supported): Pass "multiprocess+" as supported
	features.
	(remote_open_1): Clear multi_process_aware.  Set extended
	accordingly.
	(remote_detach_1): Detach current process.  Use extended packet
	format for extended-remote multi-process.  Detach process from the
	inferior list.  Only mourn after printing output.
	(extended_remote_attach_1): Add process to the inferior list.
	(remote_vcont_resume): Use write_ptid to pass the thread ids.
	(remote_wait): Use read_ptid.  Implement the extended
	multi-process extension format of the 'W' and 'X' reply packets.
	Remove exited inferiors from inferior list.
	(remote_xfer_memory): Set general thread.
	(remote_vkill): New.
	(extended_remote_kill): New.
	(remote_mourn_1): Discard all inferiors.
	(select_new_thread_callback): New.
	(extended_remote_mourn_1): If there are more processes to debug,
	switch to a thread in another process, and don't pop the target.
	(extended_remote_create_inferior_1): Add the new process to the
	inferior list.
	(remote_stopped_by_watchpoint): Indenting.
	(remote_xfer_partial): Set the general thread.
	(remote_pid_to_str): If the remote is multi-process aware, print
	the process id as well as the thread id.
	(remote_get_thread_local_address): Use write_ptid.
	(init_extended_remote_ops): Register extended_remote_kill.
	(_initialize_remote): Register new packets.  Change
	magic_null_ptid's, not_sent_ptid's and any_thread_ptid's pid
	member to 42000.

	* thread.c (thread_change_ptid): Also account for the inferior pid
	changing.

	* inferior.h (discard_all_inferiors): Declare.
	* inferior.c (discard_all_inferiors): New.