install: make UnitFileChangeType enum anonymous

install: make UnitFileChangeType enum anonymous

We almost never use the named enum type, in almost all cases we use
"int" instead, since we overload it with negative errnos. To simplify
things, let's use "int" really everywhere.

Moreover, let's rename the fields for this enum to "type_or_errno", to
make the overloading clear. And let's ad some assertions that things are
in the right range.