This patch adds support for telnet-based console servers to consolefs.

Consoles may be addressed by setting the dev attribute to the remote
machine you wish serve; see consolefs(4).

For example, a consoledb entry for kremvax, which uses port 5001 on
portserver would have the following entry:

	console=kremvax dev=tcp!portserver!5001
		gid=sys

This patch is primarily useful for integrating with non-Plan 9 console
servers and requires the telnet-notkbd patch be applied.