maint: move the last coreutils-specific files from lib/ to gl/

maint: move the last coreutils-specific files from lib/ to gl/

Convert the few remaining coreutils-specific files in lib/ to
gnulib-style modules under gl/, removing their corresponding .m4
files, since the information recorded in those files is better
stored in module-description file in gl/modules/.
* bootstrap.conf (gnulib_modules): Add new modules:
fd-reopen, buffer-lcm, xfts, strnumcmp.
* gl/lib/buffer-lcm.c: Renamed from the file in lib/.
* gl/lib/buffer-lcm.h: Likewise.
* gl/lib/fd-reopen.c: Likewise.
* gl/lib/fd-reopen.h: Likewise.
* gl/lib/strintcmp.c: Likewise.
* gl/lib/strnumcmp-in.h: Likewise.
* gl/lib/strnumcmp.c: Likewise.
* gl/lib/strnumcmp.h: Likewise.
* gl/lib/xfts.c: Likewise.
* gl/lib/xfts.h: Likewise.
* gl/modules/buffer-lcm: New module-description file.
* gl/modules/fd-reopen: Likewise.
* gl/modules/strnumcmp: Likewise.
* gl/modules/xfts: Likewise.
* m4/fd-reopen.m4: Remove, no longer needed.
* m4/strnumcmp.m4: Likewise.
* m4/xfts.m4: Likewise.
* m4/prereq.m4: Do not AC_REQUIRE the m4 functions from
our just-removed m4/*.m4 files.