fuse: add struct fuse_chan

fuse: add struct fuse_chan

The goal is to separate transport layer stuff out from struct fuse_conn,
leaving just the filesystem related members.

Add a new object referenced from fuse_conn.  This patch just implements the
allocation and freeing of this object.

Following patches will move transport related members from fuse_conn to
fuse_chan.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>