Extend and rename ometric_set_int_with_label to ometric_set_int_with_labels

Extend and rename ometric_set_int_with_label to ometric_set_int_with_labels

Instead of passing a single key value pair allow for multiple keys and values.
This is needed for rpki-client where 2 extra key value pairs are needed for
some values. To simplify passing simple values introduce a OKV() macro
which creates a compound literal array with the NULL terminal.
OK tb@