Change the seeder array from a modulo to a logical AND, improving performance

Change the seeder array from a modulo to a logical AND, improving performance
and allowing us to overflow the index variable without overflowing the
array.

Implement seeding via writes to /dev/random.  Seeding is only allowed
if the kern.seedenable sysctl is set to 1 (defaults to 0), and the securelevel
is less then or equal to 0.  Note that the random number generator also
self-seeds to some degree.

Writing to /dev/random is no longer a sink null.  Writing to /dev/urandom
is no longer allowed at all.

Modify the seeding code to mix a few of the bits with PNG feedback to reduce
the possibility of seeding-based attacks.

Intentionally feed /dev/urandom back into /dev/random for a bit, then run
DIEHARD to test for any obvious screwups.  Passes DIEHARD.