s3: Plumb smb_filename from create_file all of the way down to fd_open

s3: Plumb smb_filename from create_file all of the way down to fd_open

I used the smb_filename struct everywhere that was feasible for the
first pass.  There are still some places in this path that need to be
changed to use smb_filename, but this is a good start.

I also:
- Removed fname/path arguments from a few functions that weren't
  really using them.
- Added a utility function for detecting whether an smb_filename is a
  stream.