Exit process with correct error codes (mochajs#2445)

* Exit with code 130 in SIGINT. Refs mochajs#2438

* Exit with code 255 if more errors than 255 were returned. Fixes mochajs#2438

bin/_mocha

@@ -384,7 +384,7 @@ if (program.watch) {
   process.on('SIGINT', function() {
     showCursor();
     console.log('\n');
-    process.exit();
+    process.exit(130);
   });
 
   var watchFiles = utils.files(cwd, [ 'js' ].concat(program.watchExtensions));
@@ -442,7 +442,7 @@ if (program.watch) {
 
 function exitLater(code) {
   process.on('exit', function() {
-    process.exit(code);
+    process.exit(Math.min(code, 255));
   });
 }
 
@@ -452,7 +452,7 @@ function exit(code) {
   // https://github.com/visionmedia/mocha/issues/333 has a good discussion
   function done() {
     if (!(draining--)) {
-      process.exit(code);
+      process.exit(Math.min(code, 255));
     }
   }
 
@@ -470,6 +470,11 @@ function exit(code) {
 
 process.on('SIGINT', function() {
   runner.abort();
+
+  // This is a hack:
+  // Instead of `process.exit(130)`, set runner.failures to 130 (exit code for SIGINT)
+  // The amount of failures will be emitted as error code later
+  runner.failures = 130;
 });
 
 /**