duplicate fields removed, st_mysql_storage_engine added to support
run-time handlerton initialization (no compiler warnings), handler API
is now tied to MySQL version, handlerton->plugin mapping added
(slot-based), dummy default_hton removed, plugin-type-specific
initialization generalized, built-in plugins are now initialized too,
--default-storage-engine no longer needs a list of storage engines
in handle_options().
mysql-test-run.pl bugfixes
- changed alter tablespace truct to work with handlerton pointer (to avoid crash in parser on unknown db type"
- pushed warnings when default storage engine is used and/or tablespace stuff is not supported by storage engine
