Replace 'target async' by 'maintenance set remote-async' and

	Replace 'target async' by 'maintenance set remote-async' and
	'target remote' combination.

	* remote.c (remote_async_wait): Merge into remote_wait, and
	remove.
	(remote_async_permitted, remote_async_permitted_set): New
	variables.
	(set_maintenance_remote_async_permitted)
	(show_maintenance_remote_async_permitted): New functions.
	(remote_async_ops, extended_async_remote_ops): Delete.
	(remote_async_open, extended_remote_async_open): Delete.
	(remote_open_1): Drop async_p parameter.  Update callers.  Replace
	async_p with remote_async_permitted checks.
	(extended_async_remote_attach): Delete.
	(remote_resume, remote_async_resume): Merge and leave remote_resume.
	(remote_async_terminal_inferior): Rename to...
	(remote_terminal_inferior): ... this, and add
	remote_async_termitted check.
	(remote_async_terminal_ours): Rename to...
	(remote_terminal_ours): ... this, and add remote_async_termitted
	check.
	(remote_wait, remote_async_wait): Merge and leave remote_wait
	only.
	(remote_kill, remote_async_kill): Merge and leave remote_kill
	only.
	(remote_async_mourn, extended_async_remote_mourn): Delete.
	(extended_remote_create_inferior_1): Drop async_p parameter.
	Update callers.  Always use extended_remote_ops.
	(extended_remote_async_create_inferior): Delete.
	(remote_return_zero): Delete.
	(init_remote_ops): Register remote_can_async_p, remote_async,
	remote_async_mask, remote_terminal_inferior and
	remote_terminal_ours.
	(remote_can_async_p, remote_is_async_p): Check for
	remote_async_permitted.
	(init_remote_async_ops, init_extended_async_remote_ops): Remove.
	(set_remote_cmd): Don't add async and extended-async targets.
	(_initialize_remote): Add set/show remote-async maintenance
	commands.

	gdb/doc/
	* gdb.texinfo (-target-select): Remove reference to target async.
	(Maintenance Commands): Document "maint set/show remote-async".