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/libsystemd-network/sd-dhcp6-client.c

@@ -999,7 +999,7 @@ static int client_receive_message(
                         break;
                 }
 
-                /* fall through for Soliciation Rapid Commit option check */
+                /* fall through */ /* for Soliciation Rapid Commit option check */
         case DHCP6_STATE_REQUEST:
         case DHCP6_STATE_RENEW:
         case DHCP6_STATE_REBIND: