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.

src/basic/time-util.c

@@ -1191,7 +1191,10 @@ bool ntp_synced(void) {
         if (adjtimex(&txc) < 0)
                 return false;
 
-        if (txc.status & STA_UNSYNC)
+        /* Consider the system clock synchronized if the reported maximum error is smaller than the maximum
+         * value (16 seconds). Ignore the STA_UNSYNC flag as it may have been set to prevent the kernel from
+         * touching the RTC. */
+        if (txc.maxerror >= 16000000)
                 return false;
 
         return true;