Use Gnulib filevercmp for version comparison

Use Gnulib filevercmp for version comparison

* admin/merge-gnulib (GNULIB_MODULES): Add filevercmp.
* doc/lispref/strings.texi (Text Comparison):
* etc/NEWS, src/fns.c:
* test/src/fns-tests.el (fns-tests-string-version-lessp):
Rename newly-introduced function to string-version-lessp, by
analogy with strverscmp.
* lib/filevercmp.c, lib/filevercmp.h: New files, copied from gnulib.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* src/fns.c: Include <filevercmp.h>.
(gather_number_from_string): Remove.
(Fstring_version_lessp): Reimplement via filevercmp.