Undocumented TTY driver features

[Guy Harris]

The S3 and S5 manuals, and the S5 Interface Definition, tell you that the
TCXONC terminal "ioctl" call can be used to suspend output and restart it.
What they do *not* tell you is that it can also be used to suspend *input*
and restart it - i.e., that they can be used to control the blocking and
unblocking of input that the IXOFF mode (equivalent of TANDEM mode in
V7/4.xBSD) enables.  Add the following to the description of TCXONC:

	If "arg" is 2, suspend input; if 3, restart suspended input.

	Guy Harris