SCM_THREAD_CRITICAL_SECTION_START/END unless USE_THREADS is defined.
* ports.h: prototypes too.
* ports.c (scm_mode_bits, scm_port_mode): moved from fports.c.
* fports.h: prototype too.
* fports.c (scm_evict_ports): moved from ioext.c.
* ports.c (scm_close_port): return a boolean instead of unspecified.
throw an error if an error other than EBADF occurs.
* filesys.h: scm_close prototype.
* filesys.c (scm_close): new procedure, can close file descriptors
and ports (scsh compatible).
* ports.c (scm_flush_all_ports): SCM_PROC incorrectly allowed an
optional argument.
