(add_to_pid_list): Record the PID's status.
(linux_record_stopped_pid): Likewise. Make static.
(pull_pid_from_list): Return the saved status.
(linux_nat_handle_extended): Deleted.
(linux_handle_extended_wait): Combine with linux_nat_handle_extended.
Make static. Handle non-SIGSTOP for a new thread's first signal.
(flush_callback): Handle unexpected pending signals.
(linux_nat_wait): Update calls to changed functions.
* linux-nat.h (linux_record_stopped_pid, linux_handle_extended_wait):
Remove prototypes for newly static functions.
* gdb.threads/sigthread.c, gdb.threads/sigthread.exp: New.
