BUILD: Re-enable -Wimplicit-fallthrough

BUILD: Re-enable -Wimplicit-fallthrough

Getting rid of this warning is cleaner solved using a 'fall through' comment,
because it clarifies intent to a human reader.

This patch adjust a few places that cause -Wimplicit-fallthrough to trigger:

- Fix typos in the comment.
- Remove redundant 'no break' that trips up gcc from comment.
- Move the comment out of the block when the 'case' is completely surrounded
  by braces.
- Add comments where I could determine that the fall through was intentional.

Changes tested on

    gcc (Debian 9.3.0-13) 9.3.0
    Copyright (C) 2019 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

using

    make -j4 all TARGET=linux-glibc USE_OPENSSL=1 USE_LUA=1 USE_ZLIB=1 USE_PCRE2=1 USE_PCRE2_JIT=1 USE_GETADDRINFO=1