Start testing the rc(8) framework (beginning with *_oomprotect)

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Summary

This change adds 2 tests to make sure that the *_oomprotect variable
sets the protection against OOM killer properly within rc(8) scripts.

This is also adding the first tests for the rc(8) framework. More tests
will be added as we go.

MFC after: 2 weeks

Test Plan

A proper test is to rebuild the system with the patch and run the tests:

cd /usr/tests/libexec/rc
kyua test

A quick test for this change only is:

set -eu

cd libexec/rc/tests
objdir="$(make -V .OBJDIR)"

make

if ! sudo -i -- sh -c "cd $objdir && kyua test"; then
        sudo -i -- sh -c "cd $objdir && kyua report --results-file=LATEST --verbose"
fi

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Remove unnecessary pkills

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Make sure that *_oomprotect=all protects existing children (see https://reviews.freebsd.org/D35747).

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Update mtree. Now cd /usr/tests/libexec/rc && kyua test passes on a fresh installation without any issues

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Thanks, @des!