TODO: coding style and nbdkit code sharing ideas

Capture some longer-term ideas from the subthread at
<https://listman.redhat.com/archives/libguestfs/2023-February/030685.html

>:

  [Libguestfs] [libnbd PATCH v3 01/29]
  use space consistently in function and function-like macro invocations

(Message-Id: <20230215141158.2426855-2-lersek@redhat.com>.)

Signed-off-by: Laszlo Ersek <lersek@redhat.com>