arc4random(3): Add automatic tests.

This verifies that:
- arc4random zeroes its state and reseeds itself on fork
- arc4random reseeds itself on entropy consolidation (e.g., VM clone)
- arc4random fa

arc4random(3): Add automatic tests.

This verifies that:
- arc4random zeroes its state and reseeds itself on fork
- arc4random reseeds itself on entropy consolidation (e.g., VM clone)
- arc4random falls back to global state if it can't allocate local
state because address space limits cause mmap to fail

NOTE: This adds a new libc symbol __arc4random_global, but it's in
the reserved namespace and only used by t_arc4random, so no libc
minor bump.

PR kern/58632: getentropy(2) and arc4random(3) do not reseed on VM
fork

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