install: when disabling units, do so even if the unit is missing

In some cases there might be unit symlinks in .wants/ or .requires/
directories even though the unit is otherwise fully removed. In this
case, don't fail removal, but still remove the symlinks.

This reworks the symlink marking logic to always add unit files that we
are missing to the changes list, but proceed with any symlink removal
for them. This way we'll still generate useful hints that a unit is
missing if you invoke "systemctl disable idontexist.service", but also
still remove any link to it.

Fixes: #4995

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