sysutils/s6-rc: Update to 0.5.4.2
Also move USES below FLAVORS and FLAVOR in port Makefile as recommended by portlint.
List of changes between 0.5.4.1 and 0.5.4.2 mentioned in the ./NEWS file:
In 0.5.4.2
- Adaptation to skalibs-2.14.0.0.
- Bugfixes.
PR: 279116