Change ownership of admin socket before dropping permissions (#1336)

This is to allow access to the socket by members of the group that
permissions are dropped to.

cmd/yggdrasil/chuser_other.go

@@ -4,6 +4,6 @@ package main
 
 import "errors"
 
-func chuser(user string) error {
+func chuser(user, adminSockUrl string) error {
 	return errors.New("setting uid/gid is not supported on this platform")
 }