openssh: Append extraConfig to the generated config

Currently the extraConfig specified by the user is appended to the
beginning of the generate extraConfig. This behavior is not suitable as
any Match declarations specified in the extraConfig will break the
generated config. This is due to the way sshd parses statements after
the Match directive and groups them under the Match. Since there is no
method for closing a Match block, these must occur at the bottom of the
configuration file.