datagen.c: a quick program to generate a repeatable series of bytes.

datagen.c: a quick program to generate a repeatable series of bytes.

Useful for testing.  Note that we *don't* see the random number generator,
so every time you generate the bytes, you get the same sequence.

This is also vastly faster than /dev/urandom, since it doesn't try to be
cryptographically secure.  It generates about 200 megs/sec on my computer,
which is much faster than a disk and thus useful for testing the speed of
hashsplit.