Unify handling of ioctls FIOSETOWN/SIOCSPGRP/TIOCSPGRP and
FIOGETOWN/SIOCGPGRP/TIOCGPGRP. Do this by determining the meaning of
the ID parameter inside the sigio code. Also add cases for FIOSETOWN
and FIOGETOWN where there have been TIOCSPGRP and TIOCGPGRP before.
These changes allow removing the ID translation from sys_fcntl() and
sys_ioctl().

Idea from NetBSD

OK mpi@, claudio@

tweak previous;

Document return values.

Document sigio routines.