Merge master fuse_chan into fuse_session.

Merge master fuse_chan into fuse_session.

This is a code simplification patch.

- It confines most of the implementation channel implementation into
  fuse_loop_mt (which is its only user).

- It makes it more obvious in the code that channels are only ever used
  when using -o clone_fd and multi-threaded main loop.

- It simplies the definition of both struct fuse_session and struct
  fuse_chan.

- Theoretically it should result in (minuscule) performance
  improvements when not using -o clone_fd.

- Overall, it removes a lot more lines of source code than it adds :-).