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/asyncio02.c

@@ -151,7 +151,7 @@ main(int ac, char **av)
     int ret_val;	/* return value from testrun call */
     int eok;		/* everything is ok flag */
     int lc;             /* loop counter */
-    char *msg;          /* message returned from parse_opts */
+    const char *msg;          /* message returned from parse_opts */
     int flag_cnt;
 
     Tst_nobuf=1;