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/getgroups01.c

@@ -97,7 +97,7 @@ int
 main(int ac, char **av)
 {
     int lc;             /* loop counter */
-    char *ptr;          /* message returned from parse_opts */
+    const char *ptr;          /* message returned from parse_opts */
 
     int	i,			/* counter */
 	group,			/* return value from Getgid() call */