Fix gdb.threads/process-dies-while-detaching.exp

Fix gdb.threads/process-dies-while-detaching.exp

I noticed [1] a test bug in gdb.threads/process-dies-while-detaching.exp.
Simplified, the test code in question looks somewhat like this:

~~~
  # Detach from a process, and ensure that it exits after detaching.
  # This relies on inferior I/O.

  proc detach_and_expect_exit {test} {

      gdb_test_multiple "detach" $test ....

      set saw_prompt 0
      set saw_inf_exit 0
      while { !$saw_prompt && !$saw_inf_exit } {
          gdb_test_multiple "" $test {
              -re "exited, status=0" {
                  set saw_inf_exit 1
              }
              -re "$gdb_prompt " {
                  set saw_prompt 1
              }
          }
      }

      pass $test
  }
~~~

The bug is in the while loop's condition.  We want to make sure we see
both the inferior output and the prompt, so the loop's test should be:

   -    while { !$saw_prompt && !$saw_inf_exit } {
   +    while { !$saw_prompt || !$saw_inf_exit } {

If we just fix that, the test starts failing though, because it
exposes a couple latent problems:

- When called from test_detach_killed_outside, the parent doesn't
  print "exited, status=0", because in that case the child dies with a
  signal, and so detach_and_expect_exit times out.

  Fix it by making the parent print "signaled, sig=9" in that case,
  and have the .exp expect it.

- When testing against --target_board=native-gdbserver, sometimes we'd
  get this:

    ERROR: Process no longer exists
    ERROR: : spawn id exp9 not open
	while executing
    "expect {
    -i exp8 -timeout 220
	    -i $server_spawn_id
	    eof {
		pass $test
		wait -i $server_spawn_id
		unset server_spawn_id
	    }
	    timeout {
	       ..."
	("uplevel" body line 1)
	invoked from within
    "uplevel $body" NONE : spawn id exp9 not open

  The problem is that:

   - inferior_spawn_id and server_spawn_id are the same when testing
     with gdbserver.
   - gdbserver exits after "detach", so we get an eof for
     $inferior_spawn_id in the loop in detach_and_expect_exit.
     That's the first "ERROR: Process no longer exists".
   - and then when we reach test_server_exit, server_spawn_id
     is already closed (because server_spawn_id==inferior_spawn_id).

  To handle this, make the loop in detach_and_expect_exit use an
  indirect spawn id list and remove $inferior_spawn_id from the list
  as soon as we got the inferior output we're expecting, so that the
  "eof" is left unprocessed until we reach test_server_exit.

[1] I changed GDB in a way that should have made the test fail, but it
    didn't.

gdb/testsuite/ChangeLog:
2017-12-03  Pedro Alves  <palves@redhat.com>

	* gdb.threads/process-dies-while-detaching.c: Include <errno.h>
	and <string.h>.
	(parent_function): Print distinct messages when waitpid fails, or
	the child exits with a signal, or the child exits for an unhandled
	reason.
	* gdb.threads/process-dies-while-detaching.exp
	(detach_and_expect_exit): New 'inf_output_re' parameter and use
	it.  Wait for both inferior output and GDB's prompt.  Use an
	indirect spawn id list.
	(do_detach): New parameter 'child_exit'.  Use it to compute
	expected inferior output.
	(test_detach, test_detach_watch, test_detach_killed_outside):
	Adjust to pass down the expected child exit kind.