Add "set remote multiprocess-extensions-packet" command

Add "set remote multiprocess-extensions-packet" command

Being able to force-disable the RSP multiprocess extensions is useful
for testing.

gdb/ChangeLog:
2015-08-24  Pedro Alves  <palves@redhat.com>

	* NEWS (New commands): Mention set/show remote
	multiprocess-extensions-packet.
	* remote.c (remote_query_supported): Only tell the server to use
	the multiprocess extensions if the user hasn't force-disabled them
	with "set remote multiprocess-extensions-packet off".

gdb/doc/ChangeLog:
2015-08-24  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Remote Configuration): Document the "set/show
	remote multiprocess-extensions-packet" commands.