(x)memcoll: speedup when input is known to be NUL delimited

(x)memcoll: speedup when input is known to be NUL delimited

* lib/memcoll.c: Include stdlib.
(memcoll0) New function.
(strcoll_loop) New function, refactored for use in both memcoll
and memcoll0.
* lib/memcoll.h: Add prototype for memcoll0.
* lib/xmemcoll.c: (xmemcoll0) New function.
(collate_error) New function, refactored for use in both xmemcoll
and xmemcoll0.
* lib/xmemcoll.h: Add prototype for xmemcoll0.
* m4/memcoll.m4: add inline invocation.