QLocale doesn't align with C library functions to let us use them for system locale data

As noted in QTBUG-122216 and discussed in this review, various C library functions – nl_langinfo(), strftime(), strfmon(), maybe others – provide access to locale-relevant data that the user may have configured. However, these do not align well with the CLDR data that QLocale otherwise uses, making it difficult to work out how to incorporate it into QSystemLocale without causing an "impedance mismatch" – most of the problems centre on "is that a short format or a long format ?"

The bug mentioned above shows that (at least some) users expect QLocale::system() to agree with what the C library gives them; and the contribution whose review is linked above makes a reasonable attempt to square the circle, yet we do not have agreement on how to proceed.