* configure.in: AC_CHECK_FUNCS: add "system".
* simpos.c (scm_system): always define: use sysmissing if not
available. Check for HAVE_SYSTEM instead of _Windows (does
Windows lack system or does it have an unusable one?).
Check for error conditions -1 and 127. Use SCM_DEFER_INTS.
Let the argument be optional: if not supplied, call system(NULL).
* ports.c (scm_close_port): relax the type check from OPPORTP to
PORTP; closing a closed port is allowed.