With TIOCPKT enabled, each piece of output is preceded by a zero byte
on the PTY master. In addition, a non-zero byte is a flags field
that conveys information about changes on the pseudoterminal. This
patch implements the former, but not the latter. That is enough to
get telnetd(8) going, however. TIOCPKT support may be extended later.
Change-Id: I6ef9cc8cf1b4406147b088400fc8499684b62a30
e6dabba58b