i18n: Locale-aware quoting

[RenjiSann]

This PR brings initial support for Locale-aware quoting.
Basically, the code fetches the locale data (for now it looks at LC_ALL and LC_COLLATE, might have to fix it in the future).

It will parse the second part of the locale if existing to deduce the encoding: C.UTF-8, fr_FR.UTF-8 are UTF-8 encoded, while en_US and C default to simple ASCII (should be ISO-8859-1, but there is no support for it right now).

Disclaimer: Right now, only UTF-8 and ASCII encodings are supported. Further support will be discussed as it may be less trivial to implement.

In order to do this, I've heavily refactored the quoting_style code:

1. Moved code to a subfolder to split stuff and keep everything readable
2. Introduced a Quoter trait to factorize the interface of all quoting mechanisms

/// Common interface of quoting mecanisms.
trait Quoter {
    /// Push a valid character.
    fn push_char(&mut self, input: char);

    /// Push a sequence of valid characters.
    fn push_str(&mut self, input: &str) {
        for c in input.chars() {
            self.push_char(c);
        }
    }

    /// Push a continuous slice of invalid data wrt the encoding used to
    /// decode the stream.
    fn push_invalid(&mut self, input: &[u8]);

    /// Apply post-processing on the constructed buffer and return it.
    fn finalize(self: Box<Self>) -> Vec<u8>;
}

3. Made use of the trait to process the input through push_char() or push_invalid() depending on the encoding.
4. Patched the tests to accomodate for encoding-aware checks.

[RenjiSann]

Force pushed to add License headers and reformat (diff)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-quote is no longer failing!

[RenjiSann]

Referencing issues that could be fixed thanks to this patch:

• cksum: Investigate the display strategy of valid UTF-8 but non-ASCII filenames #7056
• ls: printing of 8-bit filenames is incorrect in most cases in LC_ALL=C #6639

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/usage_vs_getopt (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-quote is no longer failing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/printf/printf-quote is no longer failing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-quote is no longer failing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-quote is no longer failing!

[cakebaker]

@RenjiSann is it intentional that this file is in test and not somewhere in tests?

[RenjiSann]

Oops, that's a remainder of me testing stuff :/
I will open a merge request to revert it