statvfs: bind statvfs minimally.

* specs/posix_spec.yaml (statvfs): Move from here...
* specs/posix_compat_spec.yaml (statvfs): ...to here.
* specs/posix_sys_statvfs_spec.yaml (statvfs): New file. Specify
a minimal binding of statvfs(2).
* specs/specs.mk (specl_SPECS): Add
specs/posix_sys_statvfs_spec.yaml.
* ext/posix/sys/statvfs.c (Fstatvfs, Kpathvfs, Sstatvfs): Remove.
(Pstatvfs): Simplify drastically.
(luaopen_posix_sys_vfs): Add statvfs f_flags bitmasks.
* lib/posix/compat.lua (statvfs): Reimplement legacy multi-string
or table io/API over minimal C binding.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>