Add WANT_KEY_BINDINGS..
Extend our termcap handling to not use fixed buffers but instead
the new n_string object, and support ANY capability name in
*termcap* if WANT_KEY_BINDINGS is enabled.
Add the necessary support for the MLE to implement `bind' and
`unbind' commands, but the actual functionality has been split off
this changeset, because of the new [topic/wysh] sh(1)ell-style
argument quoting rules, which we really deserved for `bind'++.
Add many more builtin n_TERMCAP_QUERY constants already here and
now, and regenerate tcaps.h.