Restore time since last update on program startup

The default forced update interval is 30 days and inadyn starts the
counter from zero on startup.  If your dynamic IP address hardly ever
changes and inadyn is stopped and restarted more frequently than the
forced update interval, inadyn will never update your DDNS record.  As a
result your DDNS account is likely to expire.

Inadyn already updates the cache file with the new IP address, even when
the IP address hasn't changed.  Just use the cache file's modification
time to initialize the time since the last update.

If an embedded device without a battery backed RTC starts it's clock
from a hard coded time, such as 01-Jan-2010 or earlier for example, it
would cause this code to perform a forced update immediately. Every
reboot of the device would do the same.

Signed-off-by: Mike Fleetwood <mike.fleetwood@googlemail.com>
Signed-off-by: Joachim Nilsson <troglobit@gmail.com>