MySQL server settings: Make use of 'loose_' option prefix

Idea by one of the Gentoo MySQL maintainers. This makes MySQL skip over
unsupported options rather than fail, which is important with >=MySQL-8.

See also:
https://dev.mysql.com/doc/refman/8.0/en/option-files.html#option-file-syntax
https://mariadb.com/kb/en/configuring-mariadb-with-option-files/#option-prefixes

BUG: 421922

Thanks-to: Thomas Deutschmann <whissi@gentoo.org>
2 jobs for !49 with work/mysql8compat in 45 minutes and 45 seconds (queued for 108 minutes and 38 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed build_clazy_clang_tidy #92397

00:32:58

passed linux kf5-qt5 qt5.15 #92396

00:12:46

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