Further reduce diffs with OpenBSD's arc4random. The main functional

change here is to ensure that when a process forks after arc4random
is seeded, the parent and child don't observe the same random sequence.
OpenBSD's fix introduces some additional overhead in the form of a
getpid() call.  This could be improved upon, e.g., by setting a flag
in fork(), if it proves to be a problem.

This was discussed with secteam (simon, csjp, rwatson) in 2008, shortly
prior to my going out of town and forgetting all about it.  The conclusion
was that the problem with forks is worrisome, but it doesn't appear to
have introduced an actual vulnerability for any known programs.

The only significant remaining difference between our arc4random and
OpenBSD's is in how we seed the generator in arc4_stir().

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