BUG/MINOR: mworker: prevent incorrect values in uptime

BUG/MINOR: mworker: prevent incorrect values in uptime

Since the recent changes on the clocks, now.tv_sec is not to be used
between processes because it's a clock which is local to the process and
does not contain a real unix timestamp.  This patch fixes the issue by
using "data.tv_sec" which is the wall clock instead of "now.tv_sec'.
It prevents having incoherent timestamps.

It also introduces some checks on negatives values in order to never
displays a netative value if it was computed from a wrong value set by a
previous haproxy version.

It must be backported as far as 2.0.