kconfig/lxdialog: support resize

In all dialogs now properly catch KEY_RESIZE and take proper action.
In mconf try to behave sensibly when a dialog routine returns
-ERRDISPLAYTOOSMALL.

The original check for a screnn size of 80x19 is kept for now.
It may make sense to remove it later, but thats anyway what
much text is adjusted for.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>