fuse_lowlevel_notify_*: take struct fuse_session instead of struct fuse_chan

fuse_lowlevel_notify_*: take struct fuse_session instead of struct fuse_chan

The only struct fuse_chan that's available to the user application is
the one that is returned by fuse_mount. However, this is also
permanently available from struct fuse_session.

A later patch will therefore remove struct fuse_chan from the
public API completely. This patch prepares for this by changing the
fuse_lowlevel_notify_* functions to take a struct fuse_session
parameter instead of a struct fuse_chan parameter.