(x)memcoll: minor tweaks

* lib/memcoll.c (strcoll_loop): Prefer the style where 'const'
is after the type that it qualifies.
(memcoll0): Likewise.
* lib/memcoll.h (memcoll0): Likewise.
* lib/xmemcoll.c (collate_error, xmemcoll0): Likewise.
* lib/xmemcoll.h (xmemcoll0): Likewise.
* lib/memcoll.c (memcoll0): Correct the comment.  This function
differs from memcoll in that the NUL byte is part of the argument.
Omit the abort-checks, as performance is a real issue here.  Plus,
the checks were wrong anyway (an off-by-one error).  Omit local
variable 'diff', as it's a bit clearer that way.
* m4/memcoll.m4 (gl_MEMCOLL): Omit AC_FUNC_STRCOLL, as it's
no longer needed.