Add the setpgrp command

The setpgrp command allows to start a command in a new process group
similar to the setsid command for sessions.
Add the setsid command

This add the setsid command to run a program in a new session.
Use execvp instead of execve

We now use execvp instead of execve with setpgrp and setsid so we don't
need to specify the full path to execute a command.