Refactor: libpacemaker: Remove redundant args in rule check functions

We never use the next_change argument of eval_date_expression(), so drop
it.

We don't need the date argument in eval_rule() since we can use
data_set->now.

This allows us to make the date arguments const for the public and
internal API functions.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>