Remove OTTYDISC, NETLDISC and NTTYDISC definitions.

When I ported most applications away from <sgtty.h>, I noticed none of
them were actually using these definitions. I kept them in place,
because I didn't want to touch tools like pstat(8) and stty(1).

In preparation for the MPSAFE TTY layer, remove these definitions. This
doesn't have any impact with respect to binary compatibility (see
tty_conf.c).

We couldn now add an #error to <sys/ioctl_compat.h> when included
outside the kernel. Unfortunately, kdump's mkioctls includes this file
unconditionally.

Approved by: philip (mentor)