Update system time from file timestamps when time servers are not available

Fixes https://github.com/opentechinstitute/olsrd/issues/9 by adding rc.local file that sets system time from most recent file access timestamp in /etc.  
To test: Set the system time on a node to some point far in the past, using date -s YYYY-MM-DD hh:mm[:ss].  Confirm that the date has been set, and then reboot the node.  When the node comes up again, it should have a system time that is very close to accurate.