Rename user.clients to clientNames

This doesn't contain anything other than just the names. Make this
clearer.

upstream.go

@@ -1752,7 +1752,7 @@ func (uc *upstreamConn) appendLog(entity string, msg *irc.Message) (msgID string
 			return ""
 		}
 
-		for clientName, _ := range uc.user.clients {
+		for clientName, _ := range uc.user.clientNames {
 			uc.network.delivered.StoreID(entity, clientName, lastID)
 		}
 	}

user.go

@@ -308,7 +308,7 @@ type user struct {
 	networks        []*network
 	downstreamConns []*downstreamConn
 	msgStore        messageStore
-	clients         map[string]struct{} // indexed by client name
+	clientNames     map[string]struct{}
 
 	// LIST commands in progress
 	pendingLISTs []pendingLIST
@@ -334,7 +334,7 @@ func newUser(srv *Server, record *User) *user {
 		events:      make(chan event, 64),
 		done:        make(chan struct{}),
 		msgStore:    msgStore,
-		clients:  make(map[string]struct{}),
+		clientNames: make(map[string]struct{}),
 	}
 }
 
@@ -490,7 +490,7 @@ func (u *user) run() {
 				uc.updateAway()
 			})
 
-			u.clients[dc.clientName] = struct{}{}
+			u.clientNames[dc.clientName] = struct{}{}
 		case eventDownstreamDisconnected:
 			dc := e.dc