apt.dat parser: separately handle the first two lines; fix line counting

apt.dat parser: separately handle the first two lines; fix line counting

- The first two lines of apt.dat have their own special syntax -> handle
  them separately. Before this commit, the test "is the first character
  an 'I' or an 'A'?" was done for *every* line of apt.dat, while it is
  only relevant for the first one.

- Fix counting of the current line number (it was most of the time
  0-based so far, which could be seen in error messages, because the
  second line of apt.dat was absorbed without going through the common
  code path where lines are normally read at the top of the while loop).
  With this commit, line numbers in apt.dat are consistently counted
  starting from 1.