Log: libpe_status: improve config flag trace messages

Use pcmk__set/clear_flags_as() in set_config_flag() directly rather than the
working set wrappers, so the flag being set is stringified in its readable form
rather than already expanded as a hex literal. Example change:

Working set flags 0x00010000 ((0x00010000ULL)) for crm_simulate set by unpack_config:208

becomes

Working set flags 0x00010000 (pe_flag_startup_probes) for crm_simulate set by unpack_config:208