QLocale: only use LANGUAGE if it contradicts LC_ALL/LC_MESSAGES/LANG

If LANGUAGE specified only the language, without any script or
country, and matched the value we'd got from other environment
variables, we were throwing away their knowledge of script and
country, leading to falling back on the default script and country for
that language, which might be at odds with what other environment
variables had told us.

Changed to only use LANGUAGE if it contradicts (or extends) what we
would otherwise have used.  Clarified some comments in the process.

[ChangeLog][QLocale][Unix] When using LANGUAGE would lose information
about script or country, without changing language, use the locale
implied by LC_ALL, LC_MESSAGES or LANG.

Prompted-by: Safa AlFulaij <safa1996alfulaij@gmail.com>
Change-Id: Ie433e57ae6b995abafd05c931136cc9796494895
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>