Add 'const' qualifiers where needed to avoid compiler warnings.

* libltp/lib/tst_sig.c (tst_sig): Don't attempt to cleanup on
fatal errors.
* libltp/lib/parse_opts.c (parse_opts): Initialize allocated
string to prevent heap corruption.

winsup/testsuite/winsup.api/ltp/dup02.c

@@ -136,7 +136,7 @@ int
 main(int ac, char **av)
 {
     int lc;		/* loop counter */
-    char *msg;		/* message returned from parse_opts */
+    const char *msg;		/* message returned from parse_opts */
     int nfds = sizeof(Fds) / sizeof(int);
     int ind;