tree-wide: adjust fall through comments so that gcc is happy

gcc 7 adds -Wimplicit-fallthrough=3 to -Wextra. There are a few ways
we could deal with that. After we take into account the need to stay compatible
with older versions of the compiler (and other compilers), I don't think adding
__attribute__((fallthrough)), even as a macro, is worth the trouble. It sticks
out too much, a comment is just as good. But gcc has some very specific
requiremnts how the comment should look. Adjust it the specific form that it
likes. I don't think the extra stuff we had in those comments was adding much
value.

(Note: the documentation seems to be wrong, and seems to describe a different
pattern from the one that is actually used. I guess either the docs or the code
will have to change before gcc 7 is finalized.)

src/core/timer.c

@@ -422,7 +422,8 @@ static void timer_enter_waiting(Timer *t, bool initial) {
                                 }
                                 /* In a container we don't want to include the time the host
                                  * was already up when the container started, so count from
-                                 * our own startup. Fall through. */
+                                 * our own startup. */
+                                /* fall through */
                         case TIMER_STARTUP:
                                 base = UNIT(t)->manager->userspace_timestamp.monotonic;
                                 break;