report-fossil: Parse arguments before trying to detect drivers

This fixes ./report-fossil.py --help, which previously just croaked
with an error about not being able to detect the driver, because no .csv
files were specified on the command line.  Which isn't a great error
message when you're...trying to figure out the command line.

While at it, we also drop the requirement that filenames must end with
.csv.  The code to check for that was basically to try and distinguish
filenames from other arguments...which we don't need to do now that we
properly do argument parsing beforehand.  They still have to be .csv
files, of course, but mandating extensions is a bit unusual.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>