dnsmasq: sysupgrade hook to conditionally preserve dnsmasq.time

conditionally save dnsmasq.time across sysupgrade
dnsmasq uses /etc/dnsmasq.time as record of the last known good
system time to aid its validation of dnssec timestamps.  dnsmasq
updates the timestamp on process start/stop once it considers the system
time as valid. The timestamp file should be preserved across system
upgrade but should not be included as part of normal configuration
backups to prevent restores corrupting the current timestamp.

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>