gdb/remote.c: refactor pending fork status functions

gdb/remote.c: refactor pending fork status functions

In preparation for a following patch, refactor a few things that I did
find a bit awkward, and to make them a bit more reusable.

 - Pass an inferior to kill_new_fork_children instead of a pid.  That
   allows iterating on only this inferior's threads and avoid further
   filtering on the thread's pid.
 - Change thread_pending_fork_status to return a non-nullptr value only
   if the thread does have a pending fork status.
 - Remove is_pending_fork_parent_thread, as one can just use
   thread_pending_fork_status and check for nullptr.
 - Replace is_pending_fork_parent with is_fork_status, which just
   returns if the given target_waitkind if a fork or a vfork.  Push
   filtering on the pid to the callers, when it is necessary.

Change-Id: I0764ccc684d40f054e39df6fa5458cc4c5d1cd7b