Fixes to "related" OSes
Enhance a bit the checks for "related" OSes:
- ensure that
<clones>
and<derives-from>
must exist, instead of silently ignoring non-existing ones - extend
test_related
to also check for self-pointing<upgrades>
One issue was discovered, i.e. a self-pointing <upgrades>
in netbsd8.1
; fix it before adding the check that uncovers it.