Bug#732627: sftp DH 8192 support broken in proftpd-basic

[Till Maas]

Package: proftpd-basic
Version: 1.3.41-5+deb7u1

mod_sftp in proftpd-basic does not support diffie hellman with 8192
bits, which is used e.g. by Fedora for 256 bit symmetric keys.

There are at least two issues:
- the shipped dhparams.pem file does not contain respective params
- Buffers in proftpd are to small

References:
https://bugzilla.redhat.com/show_bug.cgi?id=1044586
http://bugs.proftpd.org/show_bug.cgi?id=4001