Provides telnet client functionality.

This class also has, through delegation, all the methods of a socket object
(by default, a TCPSocket, but can be set by the Proxy option to new()).

This provides methods such as close() to end the session and sysread() to read
data directly from the host, instead of via the waitfor() mechanism.

Note that if you do use sysread() directly when in telnet mode, you should
probably pass the output through preprocess() to extract telnet command
sequences.