evaluate: New internal helper __expr_evaluate_range

evaluate: New internal helper __expr_evaluate_range

This is used by the followup patch to evaluate a range without emitting
an error when the left value is larger than the right one.

This is done to handle time-matching such as
23:00-01:00 -- expr_evaluate_range() will reject this, but
we want to be able to evaluate and then handle this as a request
to match from 23:00 to 1am.

Signed-off-by: Ander Juaristi <a@juaristi.eus>
Signed-off-by: Florian Westphal <fw@strlen.de>