net/{netx,memnet},all: add netx.DialFunc, move memnet Network impl

This adds netx.DialFunc, unifying a type we have a bazillion other
places, giving it now a nice short name that's clickable in
editors, etc.

That highlighted that my earlier move (03b47a55c7) of stuff from
nettest into netx moved too much: it also dragged along the memnet
impl, meaning all users of netx.DialFunc who just wanted netx for the
type definition were instead also pulling in all of memnet.

So move the memnet implementation netx.Network into memnet, a package
we already had.

Then use netx.DialFunc in a bunch of places. I'm sure I missed some.
And plenty remain in other repos, to be updated later.

Updates tailscale/corp#27636

Change-Id: I7296cd4591218e8624e214f8c70dab05fb884e95
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -38,6 +38,7 @@ import (
 	"tailscale.com/net/dns"
 	"tailscale.com/net/ipset"
 	"tailscale.com/net/netaddr"
+	"tailscale.com/net/netx"
 	"tailscale.com/net/packet"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tsdial"
@@ -208,7 +209,7 @@ type Impl struct {
 	// TCP connection to another host (e.g. in subnet router mode).
 	//
 	// This is currently only used in tests.
-	forwardDialFunc func(context.Context, string, string) (net.Conn, error)
+	forwardDialFunc netx.DialFunc
 
 	// forwardInFlightPerClientDropped is a metric that tracks how many
 	// in-flight TCP forward requests were dropped due to the per-client
@@ -1457,7 +1458,7 @@ func (ns *Impl) forwardTCP(getClient func(...tcpip.SettableSocketOption) *gonet.
 	}()
 
 	// Attempt to dial the outbound connection before we accept the inbound one.
-	var dialFunc func(context.Context, string, string) (net.Conn, error)
+	var dialFunc netx.DialFunc
 	if ns.forwardDialFunc != nil {
 		dialFunc = ns.forwardDialFunc
 	} else {