detach in all-stop with threads running

detach in all-stop with threads running

A following patch will add a testcase that has a number of threads
constantly stepping over a breakpoint, and then has GDB detach the
process, while threads are running.  If we have more than one inferior
running, and we detach from just one of the inferiors, we expect that
the remaining inferior continues running.  However, in all-stop, if
GDB needs to pause the target for the detach, nothing is re-resuming
the other inferiors after the detach.  "info threads" shows the
threads as running, but they really aren't.  This fixes it.

gdb/ChangeLog:

	* infcmd.c (detach_command): Hold strong reference to target, and
	if all-stop on entry, restart threads on exit.
	* infrun.c (switch_back_to_stepped_thread): Factor out bits to ...
	(restart_stepped_thread): ... this new function.  Also handle
	trap_expected.
	(restart_after_all_stop_detach): New function.
	* infrun.h (restart_after_all_stop_detach): Declare.