Increase cipher and MAC buffer sizes, as a (somewhat educated) guess as to
what might be causing the issues reported here:

  https://forums.proftpd.org/smf/index.php/topic,11611.0.html
Merge branch 'master' into sftp-too-small-buffer
Increase buffer sizes where we write out mpints during the key exchange
process, for Bug#4112.
Merge pull request #51 from proftpd/sftp-too-small-buffer-bug4112

Bug#4112 - Failure to connect using mod_sftp sometimes due to too-small buffers