MDEV-31982 Remove check_db_name() from prepare_db_action()

- 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.

sql/sql_class.h

@@ -5356,6 +5356,8 @@ public:
   bool is_binlog_dump_thread();
 #endif
 
+  bool check_slave_ignored_db_with_error(const Lex_ident_db &db) const;
+
   /*
     Indicates if this thread is suspended due to awaiting an ACK from a
     replica. True if suspended, false otherwise.