main: Correct defer handling in main entrypoint.

This refactors the main code into a separate function that returns an
error instead of invoking os.Exit directly to ensure that the defers
that are setup prior to the exits are run as intended.

The main func will now exit with the non-zero error code when an error
is returned from the refactored separate func.

dcrpool.go

@@ -133,7 +133,9 @@ func newPool(db pool.Database, cfg *config) (*miningPool, error) {
 	return p, nil
 }
 
-func main() {
+// realMain is the real main function for dcrpool.  It is necessary to work
+// around the fact that deferred functions do not run when os.Exit() is called.
+func realMain() error {
 	// Listen for interrupt signals.
 	interrupt := make(chan os.Signal, 1)
 	signal.Notify(interrupt, signals...)
@@ -142,7 +144,7 @@ func main() {
 	// logging and configures it accordingly.
 	cfg, _, err := loadConfig()
 	if err != nil {
-		os.Exit(1)
+		return err
 	}
 	defer func() {
 		if logRotator != nil {
@@ -160,13 +162,13 @@ func main() {
 
 	if err != nil {
 		mpLog.Errorf("failed to initialize database: %v", err)
-		os.Exit(1)
+		return err
 	}
 
 	p, err := newPool(db, cfg)
 	if err != nil {
 		mpLog.Errorf("failed to initialize pool: %v", err)
-		os.Exit(1)
+		return err
 	}
 
 	if cfg.Profile != "" {
@@ -215,4 +217,12 @@ func main() {
 
 	db.Close()
 	mpLog.Info("dcrpool shut down.")
+	return nil
+}
+
+func main() {
+	// Work around defer not working after os.Exit()
+	if err := realMain(); err != nil {
+		os.Exit(1)
+	}
 }