Add reset function to servo.

This will be used with clock sanity checking.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Add clock sanity check.

Check the sanity of the synchronized clock by comparing its uncorrected
frequency with the system monotonic clock. When the measured frequency
offset is larger than the value of the sanity_freq_limit option (20% by
default), a warning message will be printed and the servo will be reset.
Setting the option to zero disables the check.

This is useful to detect when the clock is broken or adjusted by another
program.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>