store and load random seed

[xwang1498]

Currently, the random seed is only logged to stdout. It's easy to lose this, making it tough to verify already-written files later or after an interruption.

This change:

• stores and deletes the random seed as random-seed alongside the random-######## files
• falls back to this stored seed in verify/readonly mode if the user didn't manually specify a seed.

(One small downside: this change effectively disallows the 0 seed, for the sake of a simpler implementation. If that's not desired, I can fix it.)

Fixes #5 .