Make sure we don't resume the stepped thread by accident.

Say:

<stopped at a breakpoint in thread 2>
(gdb) thread 3
(gdb) step

The above triggers the prepare_to_proceed/deferred_step_ptid process,
which switches back to thread 2, to step over its breakpoint before
getting back to thread 3 and "step" it.

If while stepping over the breakpoint in thread 2, a signal arrives,
and it is set to pass/nostop, we'll set a step-resume breakpoint at
the supposed signal-handler resume address, and call keep_going.  The
problem is that we were supposedly stepping thread 3, and that
keep_going delivers a signal to thread 2, and due to scheduler-locking
off, resumes everything else, _including_ thread 3, the thread we want
stepping.  This means that we lose control of thread 3 until the next
event, when we stop everything.  The end result for the user, is that
GDB lost control of the "step".

Here's the current infrun debug output of the above, with the testcase
in the patch below:

infrun: clear_proceed_status_thread (Thread 0x2aaaab8f5700 (LWP 11663))
infrun: clear_proceed_status_thread (Thread 0x2aaaab6f4700 (LWP 11662))
infrun: clear_proceed_status_thread (Thread 0x2aaaab4f2b20 (LWP 11659))
infrun: proceed (addr=0xffffffffffffffff, signal=144, step=1)
infrun: prepare_to_proceed (step=1), switched to [Thread 0x2aaaab6f4700 (LWP 11662)]
infrun: resume (step=1, signal=0), trap_expected=1, current thread [Thread 0x2aaaab6f4700 (LWP 11662)] at 0x40098f
infrun: wait_for_inferior ()
infrun: target_wait (-1, status) =
infrun:   11659 [Thread 0x2aaaab6f4700 (LWP 11662)],
infrun:   status->kind = stopped, signal = SIGUSR1
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED
infrun: stop_pc = 0x40098f
infrun: random signal 30

Program received signal SIGUSR1, User defined signal 1.
infrun: signal arrived while stepping over breakpoint
infrun: inserting step-resume breakpoint at 0x40098f
infrun: resume (step=0, signal=30), trap_expected=0, current thread [Thread 0x2aaaab6f4700 (LWP 11662)] at 0x40098f

^^^ this is a wildcard resume.

infrun: prepare_to_wait
infrun: target_wait (-1, status) =
infrun:   11659 [Thread 0x2aaaab6f4700 (LWP 11662)],
infrun:   status->kind = stopped, signal = SIGTRAP
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED
infrun: stop_pc = 0x40098f
infrun: BPSTAT_WHAT_STEP_RESUME
infrun: resume (step=1, signal=0), trap_expected=1, current thread [Thread 0x2aaaab6f4700 (LWP 11662)] at 0x40098f

^^^ step-resume hit, meaning the handler returned, so we go back to stepping thread 3.

infrun: prepare_to_wait
infrun: target_wait (-1, status) =
infrun:   11659 [Thread 0x2aaaab6f4700 (LWP 11662)],
infrun:   status->kind = stopped, signal = SIGTRAP
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED

infrun: stop_pc = 0x40088b
infrun: switching back to stepped thread
infrun: Switching context from Thread 0x2aaaab6f4700 (LWP 11662) to Thread 0x2aaaab8f5700 (LWP 11663)
infrun: resume (step=1, signal=0), trap_expected=0, current thread [Thread 0x2aaaab8f5700 (LWP 11663)] at 0x400938
infrun: prepare_to_wait
infrun: target_wait (-1, status) =
infrun:   11659 [Thread 0x2aaaab8f5700 (LWP 11663)],
infrun:   status->kind = stopped, signal = SIGTRAP
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED
infrun: stop_pc = 0x40093a
infrun: keep going
infrun: resume (step=1, signal=0), trap_expected=0, current thread [Thread 0x2aaaab8f5700 (LWP 11663)] at 0x40093a
infrun: prepare_to_wait
infrun: target_wait (-1, status) =
infrun:   11659 [Thread 0x2aaaab8f5700 (LWP 11663)],
infrun:   status->kind = stopped, signal = SIGTRAP
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED
infrun: stop_pc = 0x40091e
infrun: stepped to a different line
infrun: stop_stepping
[Switching to Thread 0x2aaaab8f5700 (LWP 11663)]
69            (*myp) ++; /* set breakpoint child_two here */

^^^ we stopped at the wrong line.  We still stepped a bit because the
test is running in a loop, and when we got back to stepping thread 3,
it happened to be in the stepping range.  (The loop increments a
counter, and the test makes sure it increments exactly once.  Without
the fix, the counter increments a bunch, since the user-stepped thread
runs free without GDB noticing.)

The fix is to switch to the stepping thread before continuing for the
step-resume breakpoint.

gdb/
2014-02-26  Pedro Alves  <palves@redhat.com>

	PR breakpoints/16292
	* infrun.c (handle_signal_stop) <signal arrives while stepping
	over a breakpoint>: Switch back to the stepping thread.

gdb/testsuite/
2014-02-26  Pedro Alves  <pedro@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	PR breakpoints/16292
	* gdb.threads/signal-while-stepping-over-bp-other-thread.c: New
	file.
	* gdb.threads/signal-while-stepping-over-bp-other-threadexp: New
	file.
Automatic date update in version.in