alcove: run without privileges by default

The alcove binary can be run:

* as a normal user for managing pools of processes
* as a normal user for creating a user namespace
* with file capabilities set on the executable
* as a setuid binary
* by using sudo
* by using a sudo alternative like super

To accomodate all these cases, remove calling alcove using sudo. To
run under sudo:

    Port = alcove_drv:start([{exec, "sudo"}]).
Reset flag after cleaning up after child