Mark functions not returning as sudo_noreturn

We also put NOTREACHED where it applies.

lib/util/regress/hexchar/hexchar_test.c

@@ -77,5 +77,5 @@ main(int argc, char *argv[])
 	printf("%s: %d tests run, %d errors, %d%% success rate\n",
 	    getprogname(), ntests, errors, (ntests - errors) * 100 / ntests);
     }
-    exit(errors);
+    return errors;
 }