Advertise all caps, CAP DEL them on registration

... so that the JOIN/history batch takes into account all capabilities.
Without this commit for example, enabling multi-prefix after the batch
makes the client send NAMES requests for all channels, which generate
needless traffic.

downstream.go

@@ -555,6 +555,14 @@ func (dc *downstreamConn) handleCapCommand(cmd string, args []string) error {
 				return err
 			}
 		}
+		if !dc.registered && dc.capVersion >= 302 {
+			// Let downstream show everything it supports, and trim
+			// down the available capabilities when upstreams are
+			// known.
+			for k, v := range needAllDownstreamCaps {
+				dc.supportedCaps[k] = v
+			}
+		}
 
 		caps := make([]string, 0, len(dc.supportedCaps))
 		for k, v := range dc.supportedCaps {
@@ -896,6 +904,7 @@ func (dc *downstreamConn) welcome() error {
 	})
 
 	dc.updateNick()
+	dc.updateSupportedCaps()
 
 	dc.forEachUpstream(func(uc *upstreamConn) {
 		for _, ch := range uc.channels {

user.go

@@ -435,8 +435,6 @@ func (u *user) run() {
 			u.forEachUpstream(func(uc *upstreamConn) {
 				uc.updateAway()
 			})
-
-			dc.updateSupportedCaps()
 		case eventDownstreamDisconnected:
 			dc := e.dc