CONTRIB: halog: Filter input lines by date and time through timestamp

I wanted to make a graph with average answer time in nagios that takes only
the last 5 mn of the log. Filtering the log before using halog was too
slow, so I added that filter to halog.

The patch attached to this mail is a proposal to add a new option : -time
[min][:max]

The values are min timestamp and/or max timestamp of the lines to be used
for stats. The date and time of the log lines between '[' and ']' are
converted to timestamp and compared to these values.

Here is an exemple of usage :
cat /var/log/haproxy.log | ./halog -srv -H -q -time $(date --date '-5 min' +%s)
CONTRIB: halog: avoid calling time/localtime/mktime for each line

The last commit provides time-based filtering. Unfortunately, it wastes
90% of the time calling the expensive time()/localtime()/mktime()
functions.

This patch does 3 things :
  - call time()/localtime() only once to initialize the correct
    struct timeinfo ;

  - call mktime() only when the time has changed regardless of
    the current second.

  - manually add the current second to the cached result.

Doing just this is enough to multiply the parsing speed by 8.