Add support for LC_COLLATE locale.

Add support for LC_COLLATE locale.
Include hard-locale.h, linebuffer.h, memcoll.h.
(struct line): New member `buf', replacing `beg' and `lim'.
All uses changed.
(hard_LC_COLLATE): New var.
(main): Initialize it.
(get_line): Use readline to read the line,
instead of doing it by hand.
That way, we get a buffer that we can pass to memcoll.
(keycmp): Use memcoll to compare lines if hard_LC_COLLATE is nonzero.