Improves Dpkg handling of multi-line descriptions.

Previous implementation was emitting warnings for every extra
description line returned by dpkg-query for each package entry.

Now we have a more complicated parsing routine for each entry taking
account that we expect extra lines of description which we are
supressing.

Should warn normally if it encounters a badly formatted initial entry
line.
Provides package descriptions for rpm packages.

rpm has a single line SUMMARY field, so this implementation is much
simpler than for dpkg.
Removed unused fields array form Dpkg#query.
Spec fixes for aptitude/aptrpm package providers.

Updates their showformat strings to handle the new formats which include
descriptions for dpkg/rpm.