sort: -R now uses less memory on long lines with internal NULs

sort: -R now uses less memory on long lines with internal NULs

* lib/Makefile.am (libcoreutils_a_SOURCES): Remove xmemxfrm.c,
xmemxfrm.h.
* lib/memxfrm.c, lib/memxfrm.h, lib/xmemxfrm.c, lib/xmemxfrm.h: Remove.
* m4/memxfrm.m4: Likewise.
* m4/prereq.m4 (gl_PREREQ): Remove gl_MEMXFRM.
* po/POTFILES.in: Remove lib/xmemxfrm.c.
* src/sort.c: Don't include xmemxfrm.h.
(cmp_hashes): Remove.
(xstrxfrm): New function.
(compare_random): If a line contains NULs, don't create a big
buffer that contains the strxfrm output of each string in the line.
Instead, accumulate checksums and differences as we go, so that
at any one time we have to store at most the output of a single
strxfrm call when processing the line.  This removes the need for
an memxfrm function.