Genie:  Avoid IOHandle_Cast in terminal primitives
relix-kernel, Genie:  Add signal_process_group()
Genie:  Call relix::signal_process_group()

Replace send_signal_to_foreground_process_group_of_terminal() by calling
the getpgrp() primitive (which internally tests that the filehandle is a
terminal) and signal_process_group().
Genie:  Axe send_signal_to_foreground_process_group_of_terminal()
Merge branch 'signal_process_group'
Genie:  Remove IOHandle_Downcast_Traits< TerminalHandle >
Genie:  Remove obsolete IsTerminal()