ALTER TABLE should either bypass enforce-storage-engine, or mysql_upgrade
should refuse to run
Allow user to alter contents of existing table without enforcing
storage engine. However, enforce storage engine on ALTER TABLE
x ENGINE=y;
Change session only variable enforce_storage_engine to be
session variable and make sure that also global value
is used if session variable is not set.
Merge from Percona Server enforced use of a specific storage engine
authored by Stewart Smith.
Modified to be session variable and modifiable only by SUPER. Use
similar implementation as default_storage_engine.
