Implement ENVIRON and NEW-ENVIRON option support

[Kaiepi]

Fairly straightforward, just write support for these two options according to their RFCs. The only addendum I'd add is USER, DISPLAY, PRINTER, and XAUTHORITY must be the only user environment variables sent (these are the ones sent by OpenBSD's TELNET implementation) by default, with the option to enable/disable which environment variables get sent on top of that, so this library stays as nice and unopinionated as possible.