rtc: more reliable method of reading rtc for initial trim

rtc: more reliable method of reading rtc for initial trim

When chrony reads in the linux rtc for the first time to trim the system
clock, it only reads it once. As it is possible that the rtc updates
itself during the read operation, the reported rtc time could be false.
To prevent this I've added a loop that reads the rtc clock twice, if the
seconds do not match retry the two read operations. If they match you
can assume the read operation was successful.

This is based on the hwclock implementation of reading the rtc clock
from the util-linux package.