libfuse: implement readdirplus for high-level API

Reuse the old "readdir" callback, but add a flags argument, that has
FUSE_READDIR_PLUS in case this is a "plus" version.  Filesystems can safely
ignore this flag, but if they want they can add optimizations based on it:
i.e. only retrieve the full attributes in PLUS mode.

The filler function is also given a flags argument and the filesystem can
set FUSE_FILL_DIR_PLUS if all the attributes in "stat" are valid.