Fix: libcrmcommon,libpe: validate interval specs correctly

Earlier commits switched some cluster properties to allow ISO 8601 durations as
well as number-and-units values. However, the option validator was not updated.
This replaces pcmkvalid_timer() with pcmkvalid_interval_spec(). The separate
pcmk__valid_time(), which was identical except requiring the result to be at
least 5 seconds, has been removed. The only option that used it (cluster-delay)
now uses the standard validator.