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/basic/time-util.c

@@ -1271,7 +1271,7 @@ bool clock_supported(clockid_t clock) {
                 if (!clock_boottime_supported())
                         return false;
 
-                /* fall through, after checking the cached value for CLOCK_BOOTTIME. */
+                /* fall through */
 
         default:
                 /* For everything else, check properly */