Package Socket:

- fix tcp,udp,unix socket
- add socket unixgram for session less unix socket
- optmize process & code
- add some standalone test for server / client (add example server config for
  rsyslog)

Package Network/Protocol:
- add unixgram protocol

Package Errors:
- fix bug in loop to prevent circular include

socket/server/unix/error.go

@@ -32,6 +32,8 @@ package unix
 import "fmt"
 
 var (
-	ErrContextClosed = fmt.Errorf("context closed")
-	ErrServerClosed  = fmt.Errorf("server closed")
+	ErrContextClosed  = fmt.Errorf("context closed")
+	ErrServerClosed   = fmt.Errorf("server closed")
+	ErrInvalidGroup   = fmt.Errorf("invalid unix group for socket group permission")
+	ErrInvalidHandler = fmt.Errorf("invalid handler")
 )