core: disallow using '-.service' as a service name

-.service.d will become a special top level drop in so don't let it be a
usable service name (otherwise the interaction gets complicated).

src/basic/unit-name.h

@@ -58,6 +58,8 @@ static inline int unit_name_mangle(const char *name, UnitNameMangle flags, char
         return unit_name_mangle_with_suffix(name, flags, ".service", ret);
 }
 
+bool service_unit_name_is_valid(const char *name);
+
 int slice_build_parent_slice(const char *slice, char **ret);
 int slice_build_subslice(const char *slice, const char *name, char **subslice);
 bool slice_name_is_valid(const char *name);