Limit the useragent log period to the same period as the access log

Limit the useragent log period to the same period as the access log

If no explicit date range was provided on the command line, the useragent
report used to contain the content of the whole useragent.log file even if
it covered a wider period than the access log.

Now, the useragent report only covers the same period as the access log
provided they both overlap. If both logs cover distinct periods, the
useragent log is not produced.