Package apt: Information

- Implemented generic callback system for packagekit, allowing to show progress
  during offline-update to user. (Thx Oleg Solovyov mcpain@)

- (tests) Report if a test (marked XFAIL) uneXpectedly passes (XPASS).
- (tests) Run them not in %check, but in *-checkinstall subpkgs. (To break
  build-dep cycle with apt-repo-tools, whose features are required by the tests,
  but which needs to be recompiled to be linked with libapt.)
- (tests) Done more extensive testing of how apt works with "rpm" repos
  via any of the file, http(s), cdrom methods; including:
  + re-fetching if the saved complete or partial pkglist indices are corrupt
    (see https://bugzilla.altlinux.org/show_bug.cgi?id=40746#c9 );
  + the verification of the checksums of pkglist indices. (The verification
    is tested in two ways:
    * The verification of the checksum of a specific type is tested by faking
      it in the meta-data: for MD5, SHA1, SHA256, BLAKE2b and just the size.
    * Simply testing that a faked pkglist file of the same size is rejected--no
      matter which hashing algorithm is used.)
  + the verification of the checksums of rpm archives. (The verification
    is tested in two ways:
    * The verification of the checksum of a specific type is tested by faking
      it in the meta-data: for MD5, SHA1, SHA256, BLAKE2b.
    * Simply testing that a faked rpm file of the same size is rejected--no
      matter which hashing algorithm is used.)
- (source code; ABI) Reverted a lot of inessential optimizations
  from 0.5.15lorg2-alt72.
- (source code; ABI) Got rid of virtual methods with default parameters
  (because they are confusing for the programmer).
- (source code; ABI) Backported some pieces of the support for the multiplicity
  of checksum (and compression) types from apt-rpm (thx imz@):
  + the type of the compression for "pkglist" indices;
  + the type of the checksum for "pkglist" indices;
  + the type of the checksum for "rpm" archives.
- Added blake2b hash support (thx glebfm@).
- Changed file and copy download methods to always compute checksums
  (thx glebfm@).

- Cleaned up the code (thx Dmitry V. Levin ldv@; including
  quite a few commits cherry-picked from http://apt-rpm.org/scm/apt.git):
  + to avoid compilation warnings altogether and some unreliable code;
  + to avoid using any old deprecated RPM API.
- Reverted (for a while) new features with unreliable implementation introduced
  in 0.5.15lorg2-alt70 (dynamic resizing of allocated memory; some support
  for large files). Updated how the other changes look in the history
  (thx darktemplar@). (The soname has been bumped again.)
- API changes:
  + Reverted inessential optimizations that caused incompatibilities with
    the Debian API (introduced in 0.5.15lorg2-alt70).
  + Made pkgCacheFile class lazy and immutable so that it better suits
    the expectations of modern libapt clients such as PackageKit
    and so that it is less prone to memory leaks and other programming errors.
  + And changed some other things (how functions return results) to avoid
    programming errors (which lead to the NULL dereference bugs listed below).
- Fixed some recently introduced and recently discovered bugs:
  + APT now can handle packages without ARCH tag (such as gpg-pubkey, brought
    by 3rd-party packages) without a crash (thx darktemplar@ et al)
    (ALT#38381, ALT#38642).
  + Some crashes with incomplete indices (after the old apt-cdrom or for
    incompatible arch).
- Increased the default APT::Cache-Limit (up to 192M)
  to make the "out of space" failure less probable for packagekit.