Add overwrite option to pws set command

[robinkrahl]

This patch adds the --overwrite option to the pws set command and
changes the default behavior to refuse overwriting programmed slots.

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I originally suggested this change in #139 but I think I’ve changed my mind: This form of the pws set command is pretty useless. If I want to overwrite a slot I should rather use pws update. If I want to add a new slot I most likely want to use pws add. Only if I want to set a specific slot I need pws set.

At the same time, changing the default behavior is a already breaking change. Why not remove pws set completely while at it and add a --slot option to pws add if I really want to write a specific slot?

[d-e-s-o]

Yeah, I agree. Let's just remove pws set. I like the --slot suggestion.

[robinkrahl]

Do you want to remove pws set at once, or first deprecate it and remove it later?

[d-e-s-o]

Do you want to remove pws set at once, or first deprecate it and remove it later?

I don't think I necessarily kept track of every breaking change in the past and we already discussed that basically "anything goes" before 1.0. So I am fine with just removing it straight away.