tailcfg: add DERPRegion.NoMeasureNoHome, deprecate+document Avoid [cap 115]

Fixes tailscale/corp#24697

Change-Id: Ib81994b5ded3dc87a1eef079eb268906a2acb3f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg_clone.go

@@ -416,13 +416,14 @@ func (src *DERPRegion) Clone() *DERPRegion {
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _DERPRegionCloneNeedsRegeneration = DERPRegion(struct {
-	RegionID   int
-	RegionCode string
-	RegionName string
-	Latitude   float64
-	Longitude  float64
-	Avoid      bool
-	Nodes      []*DERPNode
+	RegionID        int
+	RegionCode      string
+	RegionName      string
+	Latitude        float64
+	Longitude       float64
+	Avoid           bool
+	NoMeasureNoHome bool
+	Nodes           []*DERPNode
 }{})
 
 // Clone makes a deep copy of DERPMap.