time-util: improve detection of synchronized clock

time-util: improve detection of synchronized clock

Instead of checking for the STA_UNSYNC flag in the timex status, check
the maximum error. It is updated by the kernel, increasing at a rate of
500 ppm. The maximum value is 16 seconds, which triggers the STA_UNSYNC
flag.

This follows timedatex and allows timedated to correctly detect a clock
synchronized by chronyd when configured to not synchronize the RTC.