core: rework unit name validation and manipulation logic

A variety of changes:

- Make sure all our calls distuingish OOM from other errors if OOM is
  not the only error possible.

- Be much stricter when parsing escaped paths, do not accept trailing or
  leading escaped slashes.

- Change unit validation to take a bit mask for allowing plain names,
  instance names or template names or an combination thereof.

- Refuse manipulating invalid unit name

src/dbus1-generator/dbus1-generator.c

@@ -188,7 +188,7 @@ static int add_dbus(const char *path, const char *fname, const char *type) {
         }
 
         if (service) {
-                if (!unit_name_is_valid(service, TEMPLATE_INVALID)) {
+                if (!unit_name_is_valid(service, UNIT_NAME_PLAIN|UNIT_NAME_INSTANCE)) {
                         log_warning("Unit name %s is not valid, ignoring.", service);
                         return 0;
                 }