fuse: split out filesystem part of request sending

fuse: split out filesystem part of request sending

Create a new source file: req.c and add the request sending entry
functions:

  __fuse_simple_request()
  fuse_simple_background()
  fuse_simple_notify_reply()

Introduce transport layer sending functions that are called by the
respective fs layer function:

  fuse_chan_send()
  fuse_chan_send_bg()
  fuse_chan_send_notify_reply()

Move calculation of request header fields uid, gid and pid from
fuse_get_req() and fuse_force_creads() to a new helper: fuse_fill_creds().

These fileds are now passed to the transport layer via struct fuse_args.

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