Config.pm: elaborate on disable_dsa and newer OpenSSH versions

Config.pm: elaborate on disable_dsa and newer OpenSSH versions

Newer versions of the OpenSSH server software require this line:

    PubkeyAcceptedKeyTypes +ssh-dss

be added to the sshd_config file before they will accept any
"ssh-dss" (aka dsa) public keys.

Mention that if the recommended default of `$disable_dsa = 1;` is
changed (i.e. set to 0), then the above line must be manually added
to the `$chroot/j/etc/ssh/sshd_config` file or "ssh-dss" keys will
still not be accepted.

It would be possible to grep the `sshd` binary for the string
"pubkeyacceptedkeytypes" at jail setup time and automatically add
the above line to the sshd_config if `$disable_dsa = 0;` was in
effect when the jail's initial `sshd_config` file was generated.

However, there's a very good reason that dsa keys have been disabled
in the OpenSSH server software by default -- they are widely
recognized as too insecure at this point in time.

Therefore, do add the comment explaining what has to be done to
re-enable dsa keys, but do NOT do it automatically even if dsa keys
are not disabled in the "Config.pm" file.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>