Rewrite emacs mode line (re)drawing.

Replace horizontal scrolling with real multi-line editing. This allows
the whole logical line which is being edited to be displayed over
multiple physical terminal lines.

Tested by me in the console, rxvt-unicode, screen, st, tmux, wterm,
xfce4-terminal, xterm (w/ and w/out the 'autoWrap' option enabled) and
the QEMU serial console.

ok marco@