Extended-remote fork catch

Extended-remote fork catch

This patch implements catchpoints for fork events on extended-remote
Linux targets.

Implementation appeared to be straightforward, requiring four new functions
in remote.c to implement insert/remove of fork/vfork catchpoints.  These
functions are essentially stubs that just return 0 ('success') if the
required features are enabled.  If the fork events are being reported, then
catchpoints are set and hit.

However, there are some extra issues that arise with catchpoints.

1) Thread creation reporting -- fork catchpoints are hit before the
   follow_fork has been completed.  When stopped at a fork catchpoint
   in the native implementation, the new process is not 'reported'
   until after the follow is done.  It doesn't show up in the inferiors
   list or the threads list.  However, in the gdbserver case, an
   'info threads' while stopped at a fork catchpoint will retrieve the
   new thread info from the target and add it to GDB's data structures,
   prior to the follow operations.  Because of this premature report,
   things on the GDB side eventually get very confused.

   So in remote.c:remote_update_thread_list, we check to see if there
   are any pending fork parent threads.  If there are we remove the
   related fork child thread from the thread list sent by the target.

2) Kill process before fork is followed -- on the native side in
   linux-nat.c:linux_nat_kill, there is some code to handle the case where
   a fork has occurred but follow_fork hasn't been called yet.  It does
   this by using the last status to determine if a follow is pending, and
   if it is, to kill the child task.  The use of last_status is fragile
   in situations like non-stop mode where other events may have occurred
   after the fork event.  This patch identifies a fork parent
   in remote.c:extended_remote_kill in a way similar to that used in
   thread creation reporting above.  If one is found, it kills the new
   child as well.

Tested on x64 Ubuntu Lucid, native, remote, extended-remote.  Tested the
case of killing the forking process before the fork has been followed
manually.

gdb/ChangeLog:
        * remote.c (remote_insert_fork_catchpoint): New function.
        (remote_remove_fork_catchpoint): New function.
        (remote_insert_vfork_catchpoint): New function.
        (remote_remove_vfork_catchpoint): New function.
        (pending_fork_parent_callback): New function.
        (remove_new_fork_child): New function.
        (remote_update_thread_list): Call remote_notif_get_pending_events
        and remove_new_fork_child.
        (extended_remote_kill): Kill fork child when killing the
        parent before follow_fork completes.
        (init_extended_remote_ops): Initialize target vector with
        new fork catchpoint functions.