Fixes PR port-pmax/5389.
ttyflags -a clobbers them even if they are set to `off', overriding
the 4800-bit and parity modes needed by the DEC serial kbd/mice.
* Remove old MAKEDEV code for scc and dc that tried to map tty0 and
tty01 to whichever major/minor were the two comm ports.
* Use /dev/tty[abcd] for the scc driver, in linearly increasing
minor-number order. (cf. NetBSD/sparc),
* Use /dev/ttyD[0123] for the dc705 (DZ11 clone) driver, in linearly
increasing minor-number order. (cf. NetBSD/vax).
* Add the above entires to the pmax /etc/ttys.
While we're here, fix wrong major for tun, and comment out unsupported devices.
Addresses PRs pmax/4007, pmax/4330, pmax/4367.
