Fix PR 19494: hang when killing unfollowed fork children

Fix PR 19494: hang when killing unfollowed fork children

linux_nat_kill relies on get_last_target_status to determine whether
the current inferior is stopped at a unfollowed fork/vfork event.
This is bad because many things can happen ever since we caught the
fork/vfork event...  This commit rewrites that code to instead walk
the thread list looking for unfollowed fork events, similarly to what
was done for remote.c.

New test included.  The main idea of the test is make sure that when
the program stops for a fork catchpoint, and the user kills the
parent, gdb also kills the unfollowed fork child.  Since the child
hasn't been added as an inferior at that point, we need some other
portable way to detect that the child is gone.  The test uses a pipe
for that.  The program forks twice, so you have grandparent, child and
grandchild.  The grandchild inherits the write side of the pipe.  The
grandparent hangs reading from the pipe, since nothing ever writes to
it.  If, when GDB kills the child, it also kills the grandchild, then
the grandparent's pipe read returns 0/EOF and the test passes.
Otherwise, if GDB doesn't kill the grandchild, then the pipe read
never returns and the test times out, like:

 FAIL: gdb.base/catch-fork-kill.exp: fork-kind=fork: exit-kind=kill: fork: kill parent (timeout)
 FAIL: gdb.base/catch-fork-kill.exp: fork-kind=vfork: exit-kind=kill: vfork: kill parent (timeout)

No regressions on x86_64 Fedora 20.  New test passes with gdbserver as
well.

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

	PR gdb/19494
	* linux-nat.c (kill_one_lwp): New, factored out from ...
	(kill_callback): ... this.
	(kill_wait_callback): New, factored out from ...
	(kill_wait_one_lwp): ... this.
	(kill_unfollowed_fork_children): New function.
	(linux_nat_kill): Use it.

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

	PR gdb/19494
	* gdb.base/catch-fork-kill.c: New file.
	* gdb.base/catch-fork-kill.exp: New file.