* __scm.h (SCM_DEFER_INTS, SCM_ALLOW_INTS): New definitions.

* __scm.h (SCM_DEFER_INTS, SCM_ALLOW_INTS): New definitions.
Simply lock a thread C API recursive mutex.
(SCM_NONREC_CRITICAL_SECTION_START,
SCM_NONREC_CRITICAL_SECTION_END, SCM_REC_CRITICAL_SECTION_START,
SCM_REC_CRITICAL_SECTION_END): Removed.

* eval.c: Replaced SOURCE_SECTION_START / SOURCE_SECTION_END with
direct calls to scm_rec_mutex_lock / unlock around the three calls
to scm_m_expand_body.

* eval.c, eval.h (promise_free): New function.
(scm_force): Rewritten;  Now thread-safe; Removed
SCM_DEFER/ALLOW_INTS.

* pthread-threads.h: Added partially implemented plugin interface
for recursive mutexes.  These are, for now, only intended to be
used internally within the Guile implementation.

* pthread-threads.c: New file.

* threads.c: Conditionally #include "pthread-threads.c".

* eval.c, eval.h (scm_makprom, scm_force): Rewritten to be
thread-safe;

* snarf.h (SCM_MUTEX, SCM_GLOBAL_MUTEX, SCM_REC_MUTEX,
SCM_GLOBAL_REC_MUTEX): New macros.

* eval.c, threads.c, threads.h, snarf.h: Rewrote critical section
macros---use mutexes instead.

* tags.h (SCM_IM_FUTURE): New tag.

* eval.c (scm_m_future): New primitive macro.
(SCM_CEVAL): Support futures.
(unmemocopy): Support unmemoization of futures.

* print.c (scm_isymnames): Name of future isym.