core: refuse merging on units when the unit type does not support alias

core: refuse merging on units when the unit type does not support alias

The concept of merging units exists so that we can create Unit objects for a
number of names early, and then load them only later, possibly merging units
which then turn out to be symlinked to other names. This of course only makes
sense for unit types where multiple names per unit are supported. For all
others, let's refuse the merge operation early.