Refactor: various: add convenience function for checking result success

A successful pcmk__action_result_t has both exit status CRM_EX_OK (a.k.a
PCMK_OCF_OK) and execution status PCMK_EXEC_DONE. Since checking that is
clunky, we sometimes just check exit status, which is less than ideal.

The convenience function makes it easy to check both, and improves readability.