lib/connections: Fix and optimize registry (#7996)

Registry.Get used a full sort to get the minimum of a list, and the sort
was broken because util.AddressUnspecifiedLess assumed it could find out
whether an address is IPv4 or IPv6 from its Network method. However,
net.(TCP|UDP)Addr.Network always returns "tcp"/"udp".

lib/dialer/internal.go

@@ -13,7 +13,6 @@ import (
 	"os"
 	"time"
 
-	"github.com/syncthing/syncthing/lib/util"
 	"golang.org/x/net/proxy"
 )
 
@@ -61,11 +60,6 @@ func socksDialerFunction(u *url.URL, forward proxy.Dialer) (proxy.Dialer, error)
 	return proxy.SOCKS5("tcp", u.Host, auth, forward)
 }
 
-// Sort available addresses, preferring unspecified address.
-func tcpAddrLess(i interface{}, j interface{}) bool {
-	return util.AddressUnspecifiedLess(i.(*net.TCPAddr), j.(*net.TCPAddr))
-}
-
 // dialerConn is needed because proxy dialed connections have RemoteAddr() pointing at the proxy,
 // which then screws up various things such as IsLAN checks, and "let's populate the relay invitation address from
 // existing connection" shenanigans.

lib/dialer/public.go

@@ -110,7 +110,9 @@ func DialContextReusePort(ctx context.Context, network, addr string) (net.Conn,
 		return DialContext(ctx, network, addr)
 	}
 
-	localAddrInterface := registry.Get(network, tcpAddrLess)
+	localAddrInterface := registry.Get(network, func(addr interface{}) bool {
+		return addr.(*net.TCPAddr).IP.IsUnspecified()
+	})
 	if localAddrInterface == nil {
 		// Nothing listening, nothing to reuse.
 		return DialContext(ctx, network, addr)