Create a real system account for sogo.

Ref: commit ac5c2b611a5486917427a9faf4fe285a209625ff which intends to fix bug #1470. IMHO a system account should not be available for interactive usage, and also not reside together with the user accounts in /home (which often is a remote filesystem (NFS)). Also this commit copies several opensshd-files that lives under /var/empty/sshd to /home/sogo, which clearly was not intended..

Please consider this commit to create a *real* system account, with empty home directory in /var/sogo, and /sbin/nologin for shell.