Support read-only access in megafs and file pread semantics -- random access read on an uploaded file

With these changes files can be read from the megafs fuse filesystem.  Currently data integrity is not verified.  As far as I know, its not possible to do that on an arbitrary chunk of data, only the whole file.  However the hmac is still calculated, partly as a consequence of using some of the same code as mega_session_get.

Reads can be done in parallel, however the api connection is forced to be serially accessed. Its not necessary for the data connection because a new connection is created per read.

In the future, it would be better to have a MegaFile object which keeps file metadata and data connections.

Also, it would be nice to support some kind of read-ahead, since I suspect its likely that more access will be sequential.