Refactor: libpe_status: functionize creating operation digests better
    
    Basically, this takes most of rsc_action_digest(), and puts it in a new,
    exposed function pe__calculate_digests(), for future reuse. The exposed
    function creates a new op_digest_cache_t object with calculated digests;
    rsc_action_digests() takes that and puts it in a node's digest cache.
    
    This additionally functionizes most of pe__calculate_digests(), with separate
    functions for creating each of the three digests that go into a digest object:
    the digest of all parameters, the digest of non-private parameters, and the
    digest of reload parameters.
    
    There are no changes in how the code works.