reduce unnecessary escaping in quote_output

[davidsarah]

The quote_output​ function, which is used to output possibly-Unicode strings to stdout and stderr, does more backslash-escaping than is necessary for the output to be interpreted unambiguously. In particular, it will double any backslashes in the string, which makes Windows paths look ugly.

We can reduce this by making it follow POSIX-like escaping rules: within single quotes, backslashes are not special, and the only printable character that cannot appear is a single quote. If we do have a single quote or control characters in the string, then we use double quotes with backslash-escaping.