src/format.c: use only internal functions inside formatter engine
    
    format_get_data_set() is an external interface to the formatter engine.
    In order to refactor its API, avoid its usage from the internal code.
    Rename it to format_get_data_set_int() and make format_get_data_set() a
    wrapper around it. All internal functions would need to call to
    format_get_data_set_int().