Fix a bunch of fork related regressions.

I'm seeing a ton of new FAILs in fork-related tests.  Like, these and
many more:

 +FAIL: gdb.base/disp-step-syscall.exp: vfork: continue to vfork (2nd time) (timeout)
 +FAIL: gdb.base/disp-step-syscall.exp: vfork: display/i $pc (timeout)
 ...
 -PASS: gdb.base/foll-vfork.exp: exec: vfork parent follow, through step: step
 +FAIL: gdb.base/foll-vfork.exp: exec: vfork parent follow, through step: step (timeout)
 -PASS: gdb.base/foll-vfork.exp: exec: vfork parent follow, to bp: continue to bp
 +FAIL: gdb.base/foll-vfork.exp: exec: vfork parent follow, to bp: continue to bp (timeout)
  ...
  FAIL: gdb.threads/watchpoint-fork.exp: parent: multithreaded: breakpoint (A) after the first fork (timeout)
  FAIL: gdb.threads/watchpoint-fork.exp: parent: multithreaded: watchpoint A after the first fork (timeout)
  FAIL: gdb.base/fileio.exp: System(3) call (timeout)
  FAIL: gdb.threads/watchpoint-fork.exp: parent: multithreaded: watchpoint B after the first fork (timeout)
 -PASS: gdb.base/multi-forks.exp: run to exit 2
 +FAIL: gdb.base/multi-forks.exp: run to exit 2 (timeout)
  ...
  PASS: gdb.base/watch-vfork.exp: Watchpoint on global variable (hw)
 -PASS: gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (hw)
 +FAIL: gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (hw) (timeout)
  PASS: gdb.base/watch-vfork.exp: Watchpoint on global variable (sw)
 -PASS: gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw)
 +FAIL: gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw) (timeout)

Three issues with
https://sourceware.org/ml/gdb-patches/2014-06/msg00348.html
(c077881a).

 - The inner 'status' local is shadowing the outer 'status' local,
   thus PTRACE_DETACH is never seeing the status it intends to pass on
   the inferior.

 - With that fixed, we then try to pass down the SIGTRAP that results
   from the step to the inferior.  Need to filter out signals that are
   in nopass state.

 - For software single-step archs, the current code is equivalent to:

      int status = 0;
      if (WIFSTOPPED (status))
        ptrace (PTRACE_DETACH, child_pid, 0, WSTOPSIG (status));

   ... and status == 0 is WIFEXITED, not WIFSTOPPED, so we're never
   detaching.

gdb/
2014-06-09  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_child_follow_fork): Initialize status with
	W_STOPCODE (0) instead of 0.  Remove shodowing 'status' local from
	inner block.  Only pass the signal to PTRACE_DETACH if in pass
	state.