A followup for MDEV-29672 Add MTR tests covering key and key segment …

…flags and types

Adding debug output for key and keyseg flags at ha_myisam::open() time.
So now there are three points of debug output:

1. In the very end of mysql_prepare_create_table()
2. In ha_myisam::create(), after the table2myisam() call
3. In ha_myisan::open(), after the mi_open() call

mi_create(), which is is called between 2 and 3, modifies flags for
some data types, so the output in 2 and 3 is different.