HA_AUTO_PART_KEY support for handler parent class


Docs/manual.texi:
  AUTO_INCREMENT support for MERGE
mysql-test/r/merge.result:
  AUTO_INCREMENT support for MERGE
sql/ha_myisammrg.cc:
  AUTO_INCREMENT support for MERGE
sql/ha_myisammrg.h:
  AUTO_INCREMENT support for MERGE
sql/handler.cc:
  default handler::update_auto_increment now supports HA_AUTO_PART_KEY