Rethink the whole line discipline architecture. Instead of having

Rethink the whole line discipline architecture. Instead of having
multiple switchable line disciplines, we now have a single unified
one which changes its behaviour based on option settings. Each
option setting can be suggested by the back end and/or the terminal
handler, and can be forcibly overridden by the configuration. Local
echo and local line editing are separate, independently switchable,
options.

[originally from svn r895]