Fix PR 19461: strange "info thread" behavior in non-stop

Fix PR 19461: strange "info thread" behavior in non-stop

If you have "set follow-fork child" set, then if you do "info threads"
right after a fork, and before the child reports any other event to
GDB core, you'll see:

(gdb) info threads
  Id   Target Id         Frame
* 1.1  Thread 0x7ffff7fc1740 (LWP 31875) "fork-plus-threa" (running)
  2.1  process 31879 "fork-plus-threa" Selected thread is running.
(gdb)

The "Selected thread is running." bit is a bogus error.  That was GDB
trying to fetch the current frame of thread 2.1, because the external
runnning state is "stopped", and then throwing an error because the
thread is actually running.

This actually affects all-stop + schedule-multiple as well.

The problem here is that on a fork event, GDB doesn't update the
external parent/child running states.

New comprehensive test included.  The "kill inferior 1" / "kill
inferior 2" bits also trip on PR gdb/19494 (hang killing unfollowed
fork children), which was fixed by the previous patch.

gdb/ChangeLog:
2016-01-25  Pedro Alves  <palves@redhat.com>

	PR threads/19461
	* infrun.c (handle_inferior_event_1) <fork/vfork>: Update
	parent/child running states.

gdb/testsuite/ChangeLog:
2016-01-25  Pedro Alves  <palves@redhat.com>

	PR threads/19461
	* gdb.base/fork-running-state.c: New file.
	* gdb.base/fork-running-state.exp: New file.