Generalize for other distributions

[stapelberg]

…starting with Debian derivatives such as Ubuntu.

For now, this issue tracks things which are specific to Debian, so that we can come up with a good way to make the relevant parts pluggable.

file	specifically	Debian only	Debian & derivatives only
rendermanpage.go	bySuite	-	yes
renderaux.go	bySuiteStr	-	yes
archive.go	-	-	yes
getcontents.go	-	yes	-
getpackages.go	-	yes	-
download.go	-	yes	-

[anarcat]

i documented this in the FAQ section: https://anonscm.debian.org/git/srv-manpages/debian-assets.git/tree/faq.tmpl?h=faq