Though this is an error message task, the problem was deep in the
`mysql_prepare_create_table` implementation. The problem is described as
follows:

1. `append_system_key_parts` was called before
`mysql_prepare_create_table`, though key name generation was done close to
the latest stage of the latter.
2. We can't move `append_system_key_parts` in the end, because system keys
should be appended before some checks done.
3. If the checks from `append_system_key_parts` are moved to the end of
`mysql_prepare_create_table`, then some other inappropriate errors are
issued. like `ER_DUP_FIELDNAME`.

To have key name specified in error message, name generation should be done
before the checks, which consequenced in more changes.

The final design for key initialization in `mysql_prepare_create_table`
follows. The initialization is done in three phases:
1. Calculate a total number of keys created with respect to keys ignored.
 Allocate KEY* buffer.
2. Generate unique names; calculate a total number of key parts.
 Make early checks. Allocate KEY_PART_INFO* buffer.
3. Initialize key parts, make the rest of the checks.