Replace home-grown linked-lists in FreeBSD's native target with STL lists.

Replace home-grown linked-lists in FreeBSD's native target with STL lists.

FreeBSD's native target uses linked-lists to keep track of pending fork
events and fake vfork done events.  Replace the first list with std::list
and the second with std::forward_list.

gdb/ChangeLog:

	* fbsd-nat.c (struct fbsd_fork_info): Remove.
	(fbsd_pending_children): Use std::list.
	(fbsd_remember_child): Likewise.
	(fbsd_is_child_pending): Likewise.
	(fbsd_pending_vfork_done): Use std::forward_list.
	(fbsd_add_vfork_done): Likewise.
	(fbsd_is_vfork_done_pending): Likewise.
	(fbsd_next_vfork_done): Likewise.