join: wishlist: warn (or even fail) when input is not sorted

[era+gmane]

I can't remember how many times I had wished for join(1) to at least
give a warning when input is not properly sorted.

Based on a quick look at the 5.1.2 sources, I think it should be
feasible to add a simple check to src/join.c in the function join()
but I'm not sure if it's the best or most obvious way to do it.

Perhaps even an option to make this a fatal error could be added.

Thanks in advance for considering this,

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