- Adding a new class Lex_ident_db, to store normalized database names:
  lower-cased if lower-case-table-name says so,
  and checked to be a valid database name using Lex_ident_fs::check_db_name()

- Reusing the new class in parameters to functions:
    prepare_db_action()
    mysql_create_db()
    mysql_alter_db()
    mysql_rm_db()
    mysql_upgrade_db()

This change removed two old-style check_db_name() calls.