feat: Add unix group support for XML-RPC server socket permissions

- Introduced `unixgroup` configuration for XML-RPC in
  `revpipyload.conf`.
- Updated `UnixStreamXMLRPCServer` to set socket group ownership and
  permissions.

Signed-off-by: Sven Sager <akira@narux.de>

data/etc/revpipyload/revpipyload.conf

@@ -7,7 +7,7 @@ plcworkdir_set_uid = 1
 plcprogram = program.py
 plcprogram_stop_timeout = 5
 plcprogram_watchdog = 0
-plcarguments = 
+plcarguments =
 plcuid = 1000
 plcgid = 1000
 pythonversion = 3
@@ -28,6 +28,7 @@ watchdog = 1
 xmlrpc = 1
 aclfile = /etc/revpipyload/aclxmlrpc.conf
 bindip = socket
+unixgroup = picontrol
 
 [MQTT]
 mqtt = 0
@@ -38,6 +39,6 @@ write_outputs = 0
 broker_address = localhost
 port = 1883
 tls_set = 0
-username = 
-password = 
-client_id = 
+username =
+password =
+client_id =