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>

net/dnscache/dnscache.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
+	"tailscale.com/net/netx"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
@@ -355,10 +356,8 @@ func (r *Resolver) addIPCache(host string, ip, ip6 netip.Addr, allIPs []netip.Ad
 	}
 }
 
-type DialContextFunc func(ctx context.Context, network, address string) (net.Conn, error)
-
 // Dialer returns a wrapped DialContext func that uses the provided dnsCache.
-func Dialer(fwd DialContextFunc, dnsCache *Resolver) DialContextFunc {
+func Dialer(fwd netx.DialFunc, dnsCache *Resolver) netx.DialFunc {
 	d := &dialer{
 		fwd:         fwd,
 		dnsCache:    dnsCache,
@@ -369,7 +368,7 @@ func Dialer(fwd DialContextFunc, dnsCache *Resolver) DialContextFunc {
 
 // dialer is the config and accumulated state for a dial func returned by Dialer.
 type dialer struct {
-	fwd      DialContextFunc
+	fwd      netx.DialFunc
 	dnsCache *Resolver
 
 	mu          sync.Mutex
@@ -653,7 +652,7 @@ func v6addrs(aa []netip.Addr) (ret []netip.Addr) {
 // TLSDialer is like Dialer but returns a func suitable for using with net/http.Transport.DialTLSContext.
 // It returns a *tls.Conn type on success.
 // On TLS cert validation failure, it can invoke a backup DNS resolution strategy.
-func TLSDialer(fwd DialContextFunc, dnsCache *Resolver, tlsConfigBase *tls.Config) DialContextFunc {
+func TLSDialer(fwd netx.DialFunc, dnsCache *Resolver, tlsConfigBase *tls.Config) netx.DialFunc {
 	tcpDialer := Dialer(fwd, dnsCache)
 	return func(ctx context.Context, network, address string) (net.Conn, error) {
 		host, _, err := net.SplitHostPort(address)