Bugs fixed:
- Bug #20791    valgrind errors in InnoDB
  Remove Valgrind warning of Bug #20791 : in new database
  creation, we read the doublewrite buffer magic number from
  uninitialized memory; the code worked because it was extremely
  unlikely that the memory would contain the magic number
- Bug #21784    DROP TABLE crashes 5.1.12-pre if concurrent
  queries on the table
  remove update_thd() in ::store_lock()


Also includes numerous coding style fixes, etc.  See file-level
comments for details.