wgengine, proxymap: split out port mapping from Engine to new type

(Continuing quest to remove rando stuff from the "Engine")

Updates #cleanup

Change-Id: I77f39902c2194410c10c054b545d70c9744250b0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsd/tsd.go

@@ -27,6 +27,7 @@ import (
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/net/tstun"
+	"tailscale.com/proxymap"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/magicsock"
@@ -45,7 +46,9 @@ type System struct {
 	Tun            SubSystem[*tstun.Wrapper]
 	StateStore     SubSystem[ipn.StateStore]
 	Netstack       SubSystem[NetstackImpl] // actually a *netstack.Impl
-	controlKnobs   controlknobs.Knobs
+
+	controlKnobs controlknobs.Knobs
+	proxyMap     proxymap.Mapper
 }
 
 // NetstackImpl is the interface that *netstack.Impl implements.
@@ -103,6 +106,11 @@ func (s *System) ControlKnobs() *controlknobs.Knobs {
 	return &s.controlKnobs
 }
 
+// ProxyMapper returns the ephemeral ip:port mapper.
+func (s *System) ProxyMapper() *proxymap.Mapper {
+	return &s.proxyMap
+}
+
 // SubSystem represents some subsystem of the Tailscale node daemon.
 //
 // A subsystem can be set to a value, and then later retrieved. A subsystem