Enable a subset of -Wextra

Enable a subset of -Wextra

- Includes fixes for
  - -Wimplicit-fallthrough=2
    (=3 is default but requires replacing all
     fallthrough comments)
  - -Wmissing-field-initializers
  - -Wold-style-declaration
- All other warnings that would need fixes are
  disabled for now.

Change-Id: I9ce664d073a4e6a6d433e9e6f986a5086dae8aa1
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1202
Message-Id: <20250923140854.21766-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59237558/
Signed-off-by: Gert Doering <gert@greenie.muc.de>