Add casts to have it compile on amd64 without complaining about
mismatched types.

Tested:

* AR9280, TDMA slave, amd64.
Modify release.xsl to allow proper attribution for
sponsored and/or contributed works.

This works similarly to how the subversion revision is
suffixed in release notes entries when 'revision="NNNNNN"'
is set.

The <para> tag in relnotes/article.xml can now take the
following new elements:

 - contrib: defined to what type of contribution the change
   is.  Right now, only 'vendor' or 'sponsor' are used.
   'vendor' is intended for vendor-contributed code, such as
   driver updates, etc.  'sponsor' is intended for sponsored
   work (the 'Sponsored by:' in the commit template).

 - vendor: The canonical name of the vendor.

 - sponsor: The canonical name of the sponsor.

 - vendorurl: The URL for the vendor website, if applicable.

 - sponsorurl: The URL for the sponsor website, if applicable.

If 'vendor' or 'sponsor' are set, but 'contrib' is not, nothing
is rendered.  If 'contrib' is set, but no 'vendor' or 'sponsor'
are defined, nothing is printed.  If 'vendorurl' or 'sponsorurl'
are set, the 'vendor' or 'sponsor' text is link, otherwise is
non-clickable text.

Sponsored by:	The FreeBSD Foundation