stream-ssl: Improve messages when configuring SSL if it is unsupported.

stream-ssl: Improve messages when configuring SSL if it is unsupported.

Previously, if --private-key or another option that requires SSL support
was used, but OVS was built without OpenSSL support, then OVS would fail
with an error message that the specified option was not supported.  This
confused users because it made them think that the option had been removed:
    http://openvswitch.org/pipermail/discuss/2011-April/005034.html

This commit improves the error message: OVS will now report that it was
built without SSL support.  This should be make the problem clear to users.

Reported-by: Aaron Rosen <arosen@clemson.edu>
Feature #5325.