Port file-system-info to non-Microsoft

Port file-system-info to non-Microsoft

* admin/merge-gnulib (GNULIB_MODULES): Add fsusage.
* doc/emacs/files.texi (Directories): Remove documentation of
now-obsolete directory-free-space-program and
directory-free-space-args.
* etc/NEWS: Mention change.
* etc/PROBLEMS: Slow df is no longer a problem.
* lib/fsusage.c, lib/fsusage.h, m4/fsusage.m4:
New files, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lisp/dired.el (dired-free-space-program)
(dired-free-space-args): These aliases are now obsolete.
* lisp/files.el (directory-free-space-program)
(directory-free-space-args): Now obsolete.
(get-free-disk-space): Just call file-system-info instead
of the now-obsolete directory-free-space-program.
* nt/gnulib-cfg.mk (OMIT_GNULIB_MODULE_fsusage): New macro.
* src/fileio.c: Include fsusage.h.
(blocks_to_bytes, Ffile_system_info) [!DOS_NT]: New functions.
(syms_of_fileio) [!DOS_NT]: Defsubr file-system-info.