Genie:  Factor out session_controls_pgrp()
Genie:  Factor away NewProcessGroup()
Genie:  Use tag types with dynamic_group_element

They don't require access to the filehandle class type; they merely need
to be distinct.
Genie:  Move/rename FindProcessGroup() to relix
Genie -> relix:  Move GetProcessGroupInSession()
Merge branch 'get_process_group_in_session'
Genie -> relix:  Move setsid() system call
Genie -> relix:  Move setpgid() system call
Genie -> relix:  Move _exit() system call