* threads.h: Do not include "libguile/coop-defs.h". Include

* threads.h: Do not include "libguile/coop-defs.h".  Include
"libguile/pthread-threads.h" for USE_COPT_THREADS.  Removed
(previously deprecated) C level thread API prototypes.  They are
now in the thread package specific headers, "null-threads.h" and
"pthread-threads.h".
(SCM_VALIDATE_THREAD, SCM_VALIDATE_MUTEX, SCM_VALIDATE_CONDVAR):
New.
(scm_threads_init): Removed.
(SCM_CRITICAL_SECTION_START, SCM_CRITICAL_SECTION_END,
SCM_THREAD_SWITCHING_CODE, scm_i_switch_counter,
SCM_I_THREAD_SWITCH_COUNT): Define here.
(scm_single_thread_p): Removed.
(scm_call_with_new_thread): Take two args directly instead of list
of two args.
(scm_i_thread_data, scm_i_set_thread_data, SCM_THREAD_LOCAL_DATA,
SCM_SET_THREAD_LOCAL_DATA): Define here.

* threads.c: Merged with "coop-pthreads.c".