* Makefile.am: Added iselect.c and iselect.h.

* Makefile.am: Added iselect.c and iselect.h.

* coop.c (coop_qinit): Initialize fields used by
scm_internal_select.
(coop_qget, coop_qget, coop_tmp_queue): Made global.
(coop_next_runnable_thread): If GUILE_ISELECT enabled, use
replacement in iselect.c.
(coop_mutex_lock, coop_condition_variable_wait, coop_abort,
coop_join): If GUILE_ISELECT enabled, use
coop_wait_for_runnable_thread instead of
coop_next_runnable_thread.
(usleep, sleep): New replacements for system functions if
GUILE_ISELECT is enabled.

* coop-threads.h: Declare coop_wait_for_runnable_thread.

* coop-defs.h (coop_t): Added fields used by scm_internal_select.

* filesys.c: Added #include "iselect.h".  Moved FD-macros to
iselect.h.  Implement Scheme level `select' using
scm_internal_select.  (See NEWS.)

* genio.c (scm_getc): Block with scm_internal_select.  (See NEWS.)

* init.c: Call scm_init_iselect.