Fix upstream bug 4385 and FTBFS with GCC 10
- Fix API tests compile failure with GCC 10
https://github.com/proftpd/proftpd/pull/886
- mod_sftp: When handling the 'keyboard-interactive' authentication mechanism,
as used for (e.g.) PAM, make sure to properly handle DEBUG, IGNORE,
DISCONNECT, and UNIMPLEMENTED messages, per RFC 4253
(http://bugs.proftpd.org/show_bug.cgi?id=4385)