Stop sending qTStatus if the target doesn't recognize it; add packet configuration command.

GDB currently sends a qTStatus even if the target previously replied
an empty packet to a previous qTStatus.  If the target doesn't
recognize the packet, there's no point in trying again.

The machinery we have in place is packet_ok, which has the nice side
effect of forcing one to install a configuration command/knob for the
packet in question, which is often handy when you need to debug
things, and/or emulate a target that doesn't support the packet, or even,
it can be used as workaround for the old broken kgdb's that return error
to qTSTatus instead of an empty packet.

gdb/
2013-03-28  Pedro Alves  <palves@redhat.com>

	* NEWS (New options): New section.
	(New options): Mention set/show remote trace-status-packet.
	* remote.c (PACKET_qTStatus): New enumeration value.
	(remote_get_trace_status): Skip sending qTStatus if the packet is
	disabled.  Use packet_ok.
	(_initialize_remote): Register a configuration command for
	qTStatus packet.

gdb/doc/
2013-03-28  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Remote Configuration) <set remote @var{name}-packet
	table>: Add entry for "trace-status".