Turn struct fuse_chan into an implementation detail

Turn struct fuse_chan into an implementation detail

The only struct fuse_chan that's accessible to the user application is
the "master" channel that is returned by fuse_mount and stored in struct
fuse_session.

When using the multi-threaded main loop with the "clone_fd" option, each
worker thread gets its own struct fuse_chan. However, none of these are
available to the user application, nor do they hold references to struct
fuse_session (the pointer is always null).

Therefore, any presence of struct fuse_chan can be removed
without loss of functionality by relying on struct fuse_session instead.

This reduces the number of API functions and removes a potential source
of confusion (since the new API no longer looks as if it might be
possible to add multiple channels to one session, or to share one
channel between multiple sessions).

Fixes issue #17.