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;
 }

lib/util/regress/strsplit/strsplit_test.c

@@ -113,5 +113,5 @@ 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;
 }

lib/util/regress/sudo_conf/conf_test.c

@@ -38,7 +38,7 @@ sudo_dso_public int main(int argc, char *argv[]);
 # define GROUP_SOURCE_ADAPTIVE GROUP_SOURCE_DYNAMIC
 #endif
 
-static void
+sudo_noreturn static void
 usage(void)
 {
     fprintf(stderr, "usage: %s [-v] conf_file\n", getprogname());
@@ -64,6 +64,7 @@ main(int argc, char *argv[])
 	    break;
 	default:
 	    usage();
+	    /* NOTREACHED */
 	}
     }
     argc -= optind;