Use geteuid to determine subprocess' user when launching task.

I have to run mesos-agent as root(or some user with root privilege) to isolate tasks' execution environment. For security, we 

1. chmod +s to mesos-agent and then run it as some user A(We'll ssh as user A to do some ops, but NOT every has root privilege.).
2. use --switch_user to restrict tasks' capabilities(e.g. "rm -rf /" is not allowed).

The problem is that if we set CommandInfo.User to A(the same one running mesos-agent), the check in MesosContainerizerLaunch::execute()

if(uid.get() != os::getuid().get()){
  // some code
}

will always be false. As a result, all subprocesses will run as root. 

So I suggest that we use geteuid here to replace getuid, namely

if (uid.get() != ::geteuid()){ 
  // some code 
}