kernel: refactor `InitSystem` #6099

Job	Run time
Cross compiler for s390x	2m 5s
Test in OpenBSD	1m 58s
testinstall - - ubuntu-latest	2m 20s
teststandard - - ubuntu-latest	2m 20s
testmanuals - - ubuntu-latest	2m 26s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	2m 23s
testpackages testinstall-loadall - - ubuntu-latest	2m 20s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	2m 11s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	2m 20s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	2m 6s
testmockpkg testinstall testworkspace - BOOTSTRAP_MINIMAL=yes - macos-latest	2m 20s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	2m 12s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	2m 5s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	2m 2s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug" - ubuntu-20.04	2m 15s
testmockpkg testinstall - - windows-2019	2m 6s
Send Slack notification on status change	1s
	35m 30s

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