Refactor: scheduler: Add new pe__eval_*_expr functions.
    
    These new functions all take the same input arguments - an xmlNodePtr
    and a pe_rule_eval_data_t.  This latter type holds all the parameters
    that could possibly be useful for evaluating some rule.  Most functions
    will only need a few items out of this structure.
    
    Then, implement pe_test_*_expression in terms of these new functions.