(PRIMARY KEY or UNIQUE INDEX on NOT NULL columns).

innodb-index.test: Remove the --replace_regex directives.  Replace correct
values to the "Duplicate entry" messages.  This will cause the test to
fail until we somehow return the duplicate key values to MySQL.
Add test cases for creating or dropping UNIQUE INDEX or PRIMARY KEY.

innobase_copy_index_def(): Allow the function to copy a clustered index
definition to a secondary one (UNIQUE INDEX).

innobase_create_key_def(): Check the NOT NULL flag from the correct
place.  Copy the clustered index if it is a UNIQUE INDEX.

ha_innobase::prepare_drop_index(): Refuse to drop the clustered index.