Ensure that shared substrings are handled properly when passed to

Ensure that shared substrings are handled properly when passed to
a system call or other foreign function.  Many thanks to Tim
Pierce!
* symbols.h (SCM_COERCE_SUBSTR): new macro.
* filesys.c (scm_chmod, scm_rename, scm_delete_file, scm_mkdir,
scm_rmdir, scm_opendir, scm_chdir, scm_symlink, scm_readlink,
scm_lstat), ports.c (scm_sys_make_void_port), posix.c (scm_utime,
scm_putenv, scm_setlocale, scm_mknod), stime.c (setzone,
scm_strftime), vports.c (scm_make_soft_port), backtrace.c
(scm_display_error_message): use RO macros when strings may be RO.
* error.c (scm_error_scm), filesys.c (scm_chown, scm_chmod,
scm_rename, scm_delete_file, scm_mkdir, scm_rmdir, scm_opendir,
scm_chdir, scm_symlink, scm_readlink, scm_lstat), ioext.c
(scm_freopen, scm_duplicate_port, scm_fdopen), net_db.c
(scm_gethost, scm_getnet, scm_getproto, scm_getserv), ports.c
(scm_sys_make_void_port), posix.c (scm_getgrgid, scm_utime,
scm_setlocale, scm_mknod), stime.c (setzone, scm_strptime,
scm_strftime), vports.c (scm_make_soft_port): use
SCM_COERCE_SUBSTR to make sure shared substrings are
null-terminated.