The low-level init/destroy callbacks were passed a userdata pointer
that mergerfs never consumed - the caller threaded it through
fuse_lowlevel_new only to hand it back unchanged. Remove the
parameter from the ops struct, from fuse_lowlevel_new(_common),
and from the internal fuse_ll struct; update fuse_lib_init and
fuse_lib_destroy accordingly.
Also drop fuse_reply_ioctl_iov: no caller in-tree invokes it and
the public API is defined by this vendored libfuse fork.
