BUG/MINOR: hlua: return wall-clock date, not internal date in core.now()

BUG/MINOR: hlua: return wall-clock date, not internal date in core.now()

That's hopefully the last one affected by this. It was a bit trickier
because there's the promise in the doc that the date is monotonous, so
we continue to use now-start_time as the uptime value and add it to
start_date to get the current date. It was also emphasized by commit
28360dc ("MEDIUM: clock: force internal time to wrap early after boot"),
causing core.now() to return a date of Mar 20 on Apr 27. No backport is
needed.