Moved the *duplicate C object description* warnings for function
declarations in the nitpicky mode. In nitpick mode, you can suppress
those warnings (e.g. ioctl) with::

  nitpicky = True
  nitpick_ignore = [
      ("c:func", "ioctl"),
  ]

See Sphinx documentation for the config values for ``nitpick`` and
``nitpick_ignore`` [1].

With this change all the ".. cpp:function:: int ioctl(..)" descriptions
(found in the media book) can be migrated to ".. c:function:: int
ioctl(..)", without getting any warnings. E.g.::

  .. cpp:function:: int ioctl( int fd, int request, struct cec_event *argp )

  .. c:function:: int ioctl( int fd, int request, struct cec_event *argp )

The main effect, is that we get those *CPP-types* back into Sphinx's C-
namespace and we need no longer to distinguish between c/cpp references,
when we refer a function like the ioctl.

[1] http://www.sphinx-doc.org/en/stable/config.html?highlight=nitpick#confval-nitpickySigned-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Add option 'name' to the "c:function:" directive.  With option 'name'
the ref-name of a function can be modified. E.g.::

    .. c:function:: int ioctl( int fd, int request )
       :name: VIDIOC_LOG_STATUS

The func-name (e.g. ioctl) remains in the output but the ref-name
changed from ``ioctl`` to ``VIDIOC_LOG_STATUS``. The index entry for
this function is also changed to ``VIDIOC_LOG_STATUS`` and the function
can now referenced by::

    :c:func:`VIDIOC_LOG_STATUS`
Signed-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Add a sphinx-extension to customize the sphinx c-domain.  No functional
changes right yet, just the boilerplate code.
Signed-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
[ jc: coding-style tweak ]
Signed-off-by: NJonathan Corbet <corbet@lwn.net>