Bug#295885: Shouldn't purge config files when ssh-krb5 is installed

[Per Olofsson <pelle@debian.org>]

Package: ssh
Version: 1:3.8.1p1-8.sarge.4

I have ssh-krb5 installed, so I want to purge ssh. But if I do that,
the host keys and configuration files are removed. The postrm script
should check if ssh-krb5 is installed and refrain from deleting these
files if it is.

-- 
Pelle