ioctl speed-code checking

[utzoo!henry]

Nowhere in the V7 kernel does anything ever check that the speed codes
handed to a tty ioctl are valid (i.e. in the range 0..15).  Quite by
accident, all this will do to a DH or a DZ is screw up the line in
question.  But there is obviously room for all sorts of trouble here.
The simplest fix is to change the lines in dev/tty.c/ttioccomm(), about
line 236, that read:
		tp->t_ispeed = iocb.ioc_ispeed;
		tp->t_ospeed = iocb.ioc_ospeed;
to read:
		tp->t_ispeed = iocb.ioc_ispeed&017;
		tp->t_ospeed = iocb.ioc_ospeed&017;