Extends spectate plugin

[cppcooper]

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Using DF AI code as an example I implemented an auto-unpause feature. I tested against a siege event's pause.

resolves https://github.com/DFHack/dfhack/issues/2129

[cppcooper]

I've taken well beyond simple already to mean I should try to implement the API as idiomatically as possible. So here is my crack at that, based on the lock idea lethosor gave.

To test I enabled the spectate auto-unpause feature, then unpaused the game, and ran reveal hell and could not unpause. Then did the same, but ran nopause 1 after the game loaded which unpaused the game for me and still could not unpause after reveal hell. If there is another test case I've neglected, I don't know what it is yet.

[cppcooper]

I'll try to address these (in code) before Sunday