Make build_package() sanitize file modes*

* I was debating with myself for quite a while how far to go in these
  kinds of "sensible defaults"; there will always be someone who
  doesn't want the behavior. I decided that those people shouldn't
  be using deb-pkg-tools then :-) (I wonder how long it takes though,
  before I find myself in that group of people ;-).
Normalize field names in control files (makes merging easier)