* daklib/queue.py (check_valid): allow debian-installer specific sources to have 'debian-installer' section.

away from that the first time around, since I wanted to ignore some values
that no longer matter, but now I'm reusing them so we don't get 'variable
only used once' warnings.

Doing it this way also gives us an easy way to add arbitrary headers, so 
I've added X-Debian-Package while I was there.