Get rid of warnings from the cooperative threading system.

Get rid of warnings from the cooperative threading system.
* threads.h (scm_single_thread_p, scm_yield,
scm_call_with_new_thread, scm_join_thread, scm_make_mutex,
scm_lock_mutex, scm_unlock_mutex, scm_make_condition_variable,
scm_wait_condition_variable, scm_signal_condition_variable): Add
prototypes for these Scheme-visible functions.
* coop-defs.h (coop_next_runnable_thread,
coop_wait_for_runnable_thread_now, coop_wait_for_runnable_thread):
Prototypes for these here, even though they're from iselect.c.
(coop_condition_variable_wait, coop_join): Add prototypes.
* coop-threads.c (scm_threads_free_thread, scm_threads_free_mutex,
scm_threads_free_condvar): Make these smob functions static.
* coop-threads.h (coop_init): Give this a real prototype.
* coop.c: #include <unistd.h>, if we have it, for `usleep' and `sleep'.
(coop_next_runnable_thread): No need to provide prototype; it's in
coop-defs.h.