PR remote/18965: vforkdone stop reply should indicate parent PID

PR remote/18965: vforkdone stop reply should indicate parent PID

The vforkdone stop reply misses indicating the thread ID of the vfork
parent which the event relates to:

 @cindex vfork events, remote reply
 @item vfork
 The packet indicates that @code{vfork} was called, and @var{r}
 is the thread ID of the new child process. Refer to
 @ref{thread-id syntax} for the format of the @var{thread-id}
 field.  This packet is only applicable to targets that support
 vfork events.

 @cindex vforkdone events, remote reply
 @item vforkdone
 The packet indicates that a child process created by a vfork
 has either called @code{exec} or terminated, so that the
 address spaces of the parent and child process are no longer
 shared. The @var{r} part is ignored.  This packet is only
 applicable to targets that support vforkdone events.

Unfortunately, this is not just a documentation issue.  GDBserver
is really not specifying the thread ID.  I noticed because
in non-stop mode, gdb complains:

 [Thread 6089.6089] #1 stopped.
 #0  0x0000003615a011f0 in ?? ()
 0x0000003615a011f0 in ?? ()
 (gdb) set debug remote 1
 (gdb) c
 Continuing.
 Sending packet: $QPassSignals:e;10;14;17;1a;1b;1c;21;24;25;2c;4c;#5f...Packet received: OK
 Sending packet: $vCont;c:p17c9.17c9#88...Packet received: OK
   Notification received: Stop:T05vfork:p17ce.17ce;06:40d7ffffff7f0000;07:30d7ffffff7f0000;10:e4c9eb1536000000;thread:p17c9.17c9;core:2;
 Sending packet: $vStopped#55...Packet received: OK
 Sending packet: $D;17ce#af...Packet received: OK
 Sending packet: $vCont;c:p17c9.17c9#88...Packet received: OK
   Notification received: Stop:T05vforkdone:;
 No process or thread specified in stop reply: T05vforkdone:;
 (gdb)

This is not non-stop-mode-specific, however.  Consider e.g., that in
all-stop, you may be debugging more than one process at the same time.
You continue, and both processes vfork.  So when you next get a
T05vforkdone, there's no way to tell which of the parent processes is
done with the vfork.

Tests will be added later.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-09-15  Pedro Alves  <palves@redhat.com>

	PR remote/18965
	* remote-utils.c (prepare_resume_reply): Merge
	TARGET_WAITKIND_VFORK_DONE switch case with the
	TARGET_WAITKIND_FORKED case.

gdb/doc/ChangeLog:
2015-09-15  Pedro Alves  <palves@redhat.com>

	PR remote/18965
	* gdb.texinfo (Stop Reply Packets): Explain that vforkdone's 'r'
	part indicates the thread ID of the parent process.