Adopt h2testw's file format

Now F3 and h2testw share the same file format.
Guenter Knauf has suggested this feature and
got the random-number generator used in h2testw from
Harald Bogeholz, the author of h2testw.

This change also addresses the issue that
the original random-number generator only fills
the first 32 bits of the 64 bits of the random numbers on
64-bit machines.
Besides the lower entropy on 64-bit machines, this issue was
making files generated on 32-bit and 64-bit machines incompatible.
Anselm Distelrath was the first to report this issue.