Update the Dutch translation of the "Explaining BSD" article and convert it
to a PO file. The translated strings in the PO file were manually retro-fitted from the original Dutch translation.

Entities are expanded, which seems fine for e.g. &unix; or &sunos;, but for
&tm-attrib.*; 'make po' fills in the Dutch translations as the msgid. This
means one has to either replace that with the English expansion or with their
original names. I did the latter for now.

The resulting XML file is not altered. The PO file might have some lines
concatenated, the various tools like make and poedit do not seem very
consistent nor caring about this. There are both single and double spaces
in the PO file after a full stop.