* ioext.c (scm_read_delimited_x): use RO string macros for delims.

* ioext.c (scm_read_delimited_x): use RO string macros for delims.
(scm_freopen): use RO string macros for filename and modes.
(scm_duplicate_port, scm_fdopen): use RO string macros for modes.

* posix.c (scm_getgrgid): simplify conversion of name to C string.
(scm_mknod): use RO string macros for path.

* socket.c (scm_fill_sockaddr, scm_send, scm_sendto):
use SCM_ROSTRINGP, SCM_ROCHARS, SCM_ROLENGTH.

* net_db.c (scm_gethost, scm_getnet, scm_getproto, scm_getserv):
use SCM_ROSTRINGP and SCM_ROCHARS.