debian docbook2man wrapper

debian docbook2man is a jw wrapper, with different options

Detect docbook2man as jw wrapper, used on debian.
Provide extra DOCBOOK2MAN_OPTS for this (-V header-3=...)
This does not work yet, as the XML does not parse correctly, 
but it is a start to get the ball rolling.

Note that with apt-get install docbook2x you get a docbook2x-man which works.
docbook2man is installed via apt-get install docbook-utils