Ok, concensus was to install libexpat under a "private brandname", and after
some deliberation the name "libbsdxml" was chosen since it conveys the two
most important attributes: "Private to FreeBSD" and "XML".

Add a skeleton man-page to give the credit and point for further
documentation. (If somebody wants to write a true mdoc manpage for
this I am sure both the eXpat people and I will be grateful).

(Still not connected to the build)