Merge mysql.com:/mnt/hda4/home/alik/MySQL/devel/5.1.6-release-15588

into  mysql.com:/mnt/hda4/home/alik/MySQL/devel/5.1-tree

The idea of this merge is to "synchronize" 5.1-release tree (5.1.6) and
the main 5.1 tree. A patch for BUG#15588/BUG#16621 was copied into
5.1-release from 5.0 since it is important. This patch is expected to
come into the 5.1 tree after resolving 5.0 -> 5.1 megrge logjam.