Provide only single binary xtrabackup

The goals are to:

  1. Get rid of multiple xtrabackup binaries (xtrabackup, xtrabackup_55,
  xtrabackup_56, etc.)
  2. Make the XtraBackup source tree self-contained (without having to
  download server source tarballs).
  3. Get rid of patches.

  This obviously required a lot of small changes all over the source tree,
  here’s a description of the major ones:

  - merged patches/innodb56.patch to the server tree (currently based on
    5.6.11, will be rebased as a separate step);

  - merged relevant bits from XtraDB 5.1/5.5/5.6 to be able to backup
    all versions of Percona Server with a MySQL 5.6-based binary

  - cleaned up the code, removed tons of #ifdef
    XTRADB55/XTRADB_BASED/MYSQL_VERSION_ID, which are no longer necessary
    as we are building from a single tree;

  - moved the xtrabackup subdirectory to storage/innobase/xtrabackup. This
    was the only reliable way I could find to automatically inherit InnoDB
    compiler flags;

  - kewpie is gone (unmaintained, wouldn’t work with single binary anyway)

  - innodb_int.[ch] are gone;

  - build.sh is gone (one can use regular cmake commands to build in
    various configurations)

  - AUTO_DOWNLOAD is gone

  - insane hand-written Makefiles to support all kinds of tricks to build
    with external tarballs + patches are gone

  - run.sh -c does not have any effect (but is still supported for Jenkins
    compatibility)

  - the xtrabackup_binary file in the backup directory is no longer created

  - innobackupex-1.5.1 symlink is gone

  What needs further work:

  - packaging scripts are likely broken. Should be modified to use “make
    install” (short-term), and replaced with CPack (long-term)

  - documentation needs updating to reflect user-visible changes from above

  - documentation still uses its own build system (and thus is not
    installed on “make install”)