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/libltp/lib/open_flags.c

@@ -109,7 +109,7 @@
 static char Open_symbols[512];	  /* space for openflags2symbols return value */
 
 struct open_flag_t {
-    char *symbol;
+    const char *symbol;
     int  flag;
 };