attach + target always in non-stop mode: stop all threads

attach + target always in non-stop mode: stop all threads

When running with "maint set target-non-stop on", and in all-stop
mode, nothing is stopping all threads after attaching.  vAttach in
non-stop can leave all threads running and GDB has to explicitly pause
them.

This is not visible with the native target, as in that case, attach
always stops all threads (the core re-resumes them in case of
"attach&").

In addition, it's not defined which thread manages to report the
initial attach stop, so always pick the lowest one (otherwise
multi-attach.exp regresses).

gdb/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* infcmd.c (attach_post_wait): If the target is always in non-stop
	mode, and the UI is in all-stop mode, stop all threads and pick
	the one with lowest number as current.