Mark or fix switch cases that fall through

Mark or fix switch cases that fall through

Now that C++17 introduced a special fallthrough keyword for
explicitly tagging switch cases that are supposed to fall
through, newer gcc versions also implement a feature request
from 2002 to warn about maybe unwanted fall-throughs in switch
cases in other languages (like C).

For C code, we can either add a gcc specific attribute at the
end of the switch case, or use a special comment that gcc checks
for, indicating that the fall-through behaviour is indeed
intended.

This patch adds a "/* fall-through */" comment at the end of
various case blocks to silence gcc warnings and in some cases
a break, where fall-through was probably not intended.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>