Random suffix only on collision.

0.16.0 introduced random suffixes to all machine given names
(DNS hostnames) regardless of collisions within a namespace.
This commit brings Headscale more inline with Tailscale by only
adding a suffix if the hostname will collide within the namespace.

The suffix generation differs from Tailscale.
See https://tailscale.com/kb/1098/machine-names/

protocol_common.go

@@ -150,7 +150,7 @@ func (h *Headscale) handleRegisterCommon(
 			Bool("noise", machineKey.IsZero()).
 			Msg("New machine not yet in the database")
 
-		givenName, err := h.GenerateGivenName(registerRequest.Hostinfo.Hostname)
+		givenName, err := h.GenerateGivenName(machine.Namespace.Name, machine.MachineKey, registerRequest.Hostinfo.Hostname)
 		if err != nil {
 			log.Error().
 				Caller().
@@ -374,7 +374,7 @@ func (h *Headscale) handleAuthKeyCommon(
 	} else {
 		now := time.Now().UTC()
 
-		givenName, err := h.GenerateGivenName(registerRequest.Hostinfo.Hostname)
+		givenName, err := h.GenerateGivenName(machine.Namespace.Name, MachinePublicKeyStripPrefix(machineKey), registerRequest.Hostinfo.Hostname)
 		if err != nil {
 			log.Error().
 				Caller().