apt.dat parser: various little improvements

apt.dat parser: various little improvements

- don't duplicate the line buffer (array of char as well as
  std::string), it is not useful here;
- don't corrupt (truncate) input lines longer than 2048 bytes: there is
  no such limitation in the apt.dat v1000 format spec;
- fix comments handling: the apt.dat v1000 format spec states they must
  start with *two* '#' characters.