apt-pkg/acquire-item.cc: Reject files known to be invalid (LP: #346386) (Closes:...

This commit deals with the following cases:

  - First section of index file (Packages,Sources,Translation) without Package field
  - Signed release files without GPG data (NODATA)
  - i18n/Index files without hash sums

Handling unsigned Release files is more complicated, and the example code
using indexRecords is disabled as it can reject correct Release files
without hashes. How we can reliably check unsigned Release files is
another question, and not urgent anyway, as it should have no
dramatic effect (we could check that it is a valid RFC-822 section,
but that's a bit too long to write)