refactor telnet option negotiation constants

[jquast]

Nearly all of the custom-coded telnet options you program in telnetsrvlib.py can be imported from the standard python module, telnetlib, and make the beginning of your program much briefer and less daunting to a first-opener.

See for example https://github.com/jquast/x84/blob/master/x84/telnet.py#L47