Updates Channel safely

[cppcooper]

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I've been working on cave-in prevention for several days now, and preliminary testing indicates this achieves it as well as fixing some issues with insta-digging inaccessible designations.

[cppcooper]

I dropped the insta-dig feature. The dig-now command doesn't seem to work reliably, and trying my own variant didn't pan out. I left the code in (but always have the feature disabled; and not listed in documentation/status) on the chance I work up the willpower to debug it some more. At this point it isn't clear if there is a problem in channel-safely with regards to when it attempts to dig-now, or if there is only a problem with dig-now and the designations listed as jobs only.