Merge threads directory into libguile.

Merge threads directory into libguile.
* coop-defs.h, coop-threads.c, coop-threads.h, coop.c, threads.c,
threads.h: New source files.
* Makefile.am (EXTRA_libguile_la_SOURCES): Add threads.c.
(noinst_HEADERS): Add coop-threads.c, coop-threads.h, coop.c
here; see comment.
(modinclude_HEADERS): Add threads.h, coop-defs.h.
(EXTRA_DIST): Add fsu-pthreads.h, mit-pthreads.c, mit-pthreads.h,
coop-threads.c.cygnus, coop-threads.h.cygnus.
* configure.in: If we're using threads, include threads.o in
LIBOBJS.
* _scm.h, libguile.h: threads.h lives in this directory now.
* fsu-pthreads.h, mit-pthreads.c, mit-pthreads.h,
coop-threads.c.cygnus, coop-threads.h.cygnus: New files, not
currently used, but brought along for information's sake.
* ChangeLog-threads: log from old 'threads' directory.
* Makefile.in, configure: Rebuilt.