zshrc, keys: Reorganise keyboard setup

This commit causes zsh to use smkx mode while its line editor (zle) is
active. This might break backwards compatibility with older
customisations, that rely on binding hardcoded escape sequences to
widgets.

Doing this is _absolutely_ required to make the values from $terminfo
valid. Those might otherwise be wrong and absolutely useless.

We do turn off smkx mode again when the line editor is done. So, nothing
else should be affected.

This commit also reorganises much of the setup, because different
aspects were addressed in between one another.

Signed-off-by: Frank Terbeck <ft@grml.org>