core: turn unit_load_fragment_and_dropin_optional() into a flag

unit_load_fragment_and_dropin() and unit_load_fragment_and_dropin_optional()
are really the same, with one minor difference in behaviour. Let's drop
the second function.

"_optional" in the name suggests that it's the "dropin" part that is optional.
(Which it is, but in this case, we mean the fragment to be optional.)
I think the new version with a flag is easier to understand.

src/core/timer.c

@@ -178,7 +178,7 @@ static int timer_load(Unit *u) {
         assert(u);
         assert(u->load_state == UNIT_STUB);
 
-        r = unit_load_fragment_and_dropin(u);
+        r = unit_load_fragment_and_dropin(u, true);
         if (r < 0)
                 return r;