OpenSSH: allow VersionAddendum to be used again

OpenSSH: allow VersionAddendum to be used again

Prior to this, setting VersionAddendum will be a no-op: one will
always have BASE_VERSION + " " + VERSION_HPN for VersionAddendum
set in the config and a bare BASE_VERSION + VERSION_HPN when there
is no VersionAddendum is set.

HPN patch requires both parties to have the "hpn" inside their
advertized versions, so we add VERSION_HPN to the VERSION_BASE
if HPN is enabled and omitting it if HPN is disabled.

VersionAddendum now uses the following logics:
 * unset (default value): append " " and VERSION_ADDENDUM;
 * VersionAddendum is set and isn't empty: append " "
   and VersionAddendum;
 * VersionAddendum is set and empty: don't append anything.

Approved by: des
Reviewed by: bz

git-svn-id: svn://svn.freebsd.org/base/stable/9@236520 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f