suboption parsing: consume until IAC SE

[cfcs]

This is pretty unclear in the spec, but after receiving IAC SB kind you must continue parsing the suboption negotiation parameters until you see IAC SE.

The current code consumes smaller fixed-size tuples (EatSub of int * ....).

For an example of where this longer form is used, see https://tools.ietf.org/html/rfc1184#section-2.3

I wrote a hacky patch here, I'm not certain that it's correct, but at least I get less junk on the display with this :) https://github.com/cfcs/flathead/commit/e2091c7b191559813cf21a33112d1d114319af43#diff-c2b4b28d4f181563dfe33ebb2e5d85deR118-R161