Remove arbitrary limit on filenames

The filenames were from 0001.fff to 9999.fff.
This limit hasn't been a problem, but Anselm Distelrath reported
on February 8th, 2013 that isn't enough to test his system:
a RAID 6 with 18 hard drives of 3TB each.

Now filenames are from 1.fff to <INT_MAX>.fff.
Add option parameter --start-at=NUM to F3

This parameter became important because F3 now can be used on
very large storages and one may want to resume the test process
from a failure or interruption.
Increment version to 2.2