tailcfg: add IsZero methods to UserID and NodeID

These will be helpful for doing some automated refactoring.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tailcfg/tailcfg.go

@@ -27,10 +27,18 @@ type ID int64
 
 type UserID ID
 
+func (u UserID) IsZero() bool {
+	return u == 0
+}
+
 type LoginID ID
 
 type NodeID ID
 
+func (u NodeID) IsZero() bool {
+	return u == 0
+}
+
 type GroupID ID
 
 type RoleID ID