misc: fix ggc-7 fallthrough warnings

misc: fix ggc-7 fallthrough warnings

(Original patch and commit message edited by Rudi.)

gcc-7 adds -Wimplicit-fallthrough=3 to our default flag -Wextra.
This warning can be silenced by using comment /* fallthrough */
which is also recognized by other tools like coverity. There are
also other valid comments (see man gcc-7) but we consolidate this
style now.

We could have also used __attribute__((fallthrough)) but the comment
looks nice and does not need to be ifdef'ed for compatibility.

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=7652
Reference: https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/
Reviewed-by: Ruediger Meier <ruediger.meier@ga-group.nl>
Suggested-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>