tailcfg, net/netcheck: let control mark "Avoid" bit on DERP regions

So a region can be used if needed, but won't be STUN-probed or used as
its home.

This gives us another possible debugging mechanism for #1310, or can
be used as a short-term measure against DERP flip-flops for people
equidistant between regions if our hysteresis still isn't good enough.

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

tailcfg/derpmap.go

@@ -55,6 +55,12 @@ type DERPRegion struct {
 	// "San Francisco", "Singapore", "Frankfurt", etc.
 	RegionName string
 
+	// Avoid is whether the client should avoid picking this as its home
+	// region. The region should only be used if a peer is there.
+	// Clients already using this region as their home should migrate
+	// away to a new region without Avoid set.
+	Avoid bool `json:",omitempty"`
+
 	// Nodes are the DERP nodes running in this region, in
 	// priority order for the current client. Client TLS
 	// connections should ideally only go to the first entry