doc-rst:c-domain: ref-name of a function declaration

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: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>