control/controlclient: use more direct way of getting the MagicDNS suffix

Suggested by Avery earlier. Ends up fixing bug in "tailscale status" when
MagicDNS if off too:
https://forum.tailscale.com/t/1-3-293-is-released-a-1-4-0-pre-release/349/11?u=bradfitz

control/controlclient/netmap.go

@@ -18,7 +18,6 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/wgkey"
-	"tailscale.com/util/dnsname"
 	"tailscale.com/wgengine/filter"
 )
 
@@ -63,27 +62,16 @@ type NetworkMap struct {
 	// TODO(crawshaw): Capabilities []tailcfg.Capability
 }
 
-// MagicDNSSuffix returns the domain's MagicDNS suffix, or empty if none.
-// If non-empty, it will neither start nor end with a period.
+// MagicDNSSuffix returns the domain's MagicDNS suffix (even if
+// MagicDNS isn't necessarily in use).
+//
+// It will neither start nor end with a period.
 func (nm *NetworkMap) MagicDNSSuffix() string {
-	searchPathUsedAsDNSSuffix := func(suffix string) bool {
-		if dnsname.HasSuffix(nm.Name, suffix) {
-			return true
-		}
-		for _, p := range nm.Peers {
-			if dnsname.HasSuffix(p.Name, suffix) {
-				return true
-			}
-		}
-		return false
-	}
-
-	for _, d := range nm.DNS.Domains {
-		if searchPathUsedAsDNSSuffix(d) {
-			return strings.Trim(d, ".")
-		}
+	name := strings.Trim(nm.Name, ".")
+	if i := strings.Index(name, "."); i != -1 {
+		name = name[i+1:]
 	}
-	return ""
+	return name
 }
 
 func (nm *NetworkMap) String() string {

ipn/ipnstate/ipnstate.go

@@ -25,10 +25,16 @@ import (
 
 // Status represents the entire state of the IPN network.
 type Status struct {
-	BackendState   string
-	TailscaleIPs   []netaddr.IP // Tailscale IP(s) assigned to this node
-	Self           *PeerStatus
-	MagicDNSSuffix string // e.g. "userfoo.tailscale.net" (no surrounding dots)
+	BackendState string
+	TailscaleIPs []netaddr.IP // Tailscale IP(s) assigned to this node
+	Self         *PeerStatus
+
+	// MagicDNSSuffix is the network's MagicDNS suffix for nodes
+	// in the network such as "userfoo.tailscale.net".
+	// There are no surrounding dots.
+	// MagicDNSSuffix should be populated regardless of whether a domain
+	// has MagicDNS enabled.
+	MagicDNSSuffix string
 
 	Peer map[key.Public]*PeerStatus
 	User map[tailcfg.UserID]tailcfg.UserProfile