Store the max_space_id in the data dictionary header in order to avoid
space_id reuse.

DICT_HDR_MIX_ID: Renamed to DICT_HDR_MAX_SPACE_ID, DICT_HDR_MIX_ID_LOW.

dict_hdr_get_new_id(): Return table_id, index_id, space_id or a subset of them.

fil_system_t: Add ibool space_id_reuse_warned.

fil_create_new_single_table_tablespace(): Get the space_id from the caller.

fil_space_create(): Issue a warning if the fil_system->max_assigned_id
is exceeded.

fil_assign_new_space_id(): Return TRUE/FALSE and take a pointer to the
space_id as a parameter. Make the function public.

fil_init(): Initialize all fil_system fields by mem_zalloc(). Remove
explicit initializations of certain fields to 0 or NULL.