usage() has been made a (non-void) function so that it can be
used in a pointer expression (see macro `next'). Widen the
implied integer return type of usage() so that we can cast to
a pointer without warnings.
While i was at it, don't export \E[OA style cursor key strings into the
TERMCAP variable since window's emulation doesn't support it. Instead,
convert to \E[[A etc.
