Correct backspace for multibyte characters

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Since we use a very low-level password entry we need to take care of backspaces, kill characters, etc; but also this. This function conservatively only deals with UTF8 (and non-UTF8 sequences will simply continue to be seen as individual bytes).

This same behaviour is in ogsudo; if we agree on this fix I'll attempt to contribute it back there as well.

Note that the fix does not use the current locale, since the encoding that a password arrives in is determined by the terminal the user has than the local LC_CTYPE setting. And normally a mismatch between that is noticeable, but not so much in the password prompt since that is silent.

So the solution is that the multibyte detection is heuristic in the sense that:

• Correct UTF8 input will be handled correctly.
• All likely single-byte input will be handled correctly (e.g. påsswørd if it is in ISO8859-1), except if it contains mojibake such as pȧsswørd (but then the most likely intent of the user was to indeed pȧsswørd)
• Output that mixes Latin1 and non-UTF8: the heuristic doesn't look at the count bits of the UTF8 lead byte; we could add that but at the same time, in this scenario someone deserves to have unpredictable behaviour when typing backspace on the password prompt.
• I believe the only other relevant multibyte encoding is EUC, but that is, afaict, really on its way out. But we could add support for that encoding format in the similar way if needed.

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This can be reviewed commit by commit; the first has the most churn but should be a neutral change; the second is neutral as well (but the final change necessary in read_unbuffered to show that the logical changes are indeed very minor)

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I've also compared systemd-ask-password and even if I put everything into ISO8859-1, typing in some special characters will just disable the bulletprinter from that point onward; the above routine handles that more gracefully.

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This of course made me curious, e.g. for EUC-JP: even if it is being used, would it be used for writing passwords? This Reddit thread seems to suggest not.