[golangci-lint]: Use constant for chat.

const.go

@@ -20,5 +20,6 @@ const (
 	nsXMPPStanzas    = "urn:ietf:params:xml:ns:xmpp-stanzas"
 	oxAltBody        = "This message is encrypted (XEP-0373: OpenPGP for XMPP)."
 	pubsubPubOptions = "http://jabber.org/protocol/pubsub#publish-options"
-	strGroupchat	 = "groupchat"
+	strChat          = "chat"
+	strGroupchat     = "groupchat"
 )

main.go

@@ -404,7 +404,7 @@ func main() {
 	}
 
 	var msgType string
-	msgType = "chat"
+	msgType = strChat
 	if *flagChatroom {
 		msgType = strGroupchat
 		// Join the MUCs.
@@ -487,7 +487,7 @@ func main() {
 				}
 				var bareFrom string
 				switch v.Type {
-				case "chat":
+				case strChat:
 					bareFrom = strings.Split(v.Remote, "/")[0]
 				case strGroupchat:
 					bareFrom = v.Remote
@@ -519,7 +519,7 @@ func main() {
 				}
 				var bareFrom string
 				switch v.Type {
-				case "chat":
+				case strChat:
 					bareFrom = strings.Split(v.Remote, "/")[0]
 				case strGroupchat:
 					bareFrom = v.Remote