key name "PRIMARY".  If the table has a generated clustered index,
also check for a UNIQUE INDEX containing no NULL columns.

innobase_create_index_def(): Add the parameter key_primary.

innobase_create_key_def(): Check if the first index is a UNIQUE index
consisting of NOT NULL columns.

Fixing this bug revealed another one: FOREIGN KEY constraints
will be lost when a PRIMARY KEY is created.  This is the reason why
innodb.test will fail.