RCG: irigb skew generates an error when it's below 2 usec

For the status word, the new error level only applies to the LIGO PCIe timing card. Otherwise, the lower limit remains unchanged. But the displayed value only turns red below 2 no matter the timing, since MEDM is generated without knowledge of the timing system, and we favor O4 usage at the sites above backward correctness.

'IRIG-B' label on GDS-TP screen changed to 'TIME 0'