shared/install: make install_changes_add propagate passed-in errno value
    
    The function was written to only return an error from internal allocation
    failures, because when using it to create a bus message, we want to distinguish
    a failed operation from an allocation error when sending the reply. But it
    turns out that the only caller that makes this distinction checks that the
    passed-in errno value ('type') is not negative beforehand. So we can make the
    function pass 'type' value through, which makes most of the callers nicer.
    
    No functional change.