Reenable s_server -dhparam option

This option was lost when converting to a table-driven option parser
in commit 7e1b7485.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4398)

apps/s_server.c

@@ -795,6 +795,7 @@ const OPTIONS s_server_options[] = {
     {"pass", OPT_PASS, 's', "Private key file pass phrase source"},
     {"dcert", OPT_DCERT, '<',
      "Second certificate file to use (usually for DSA)"},
+    {"dhparam", OPT_DHPARAM, '<', "DH parameters file to use"},
     {"dcertform", OPT_DCERTFORM, 'F',
      "Second certificate format (PEM or DER) PEM default"},
     {"dkey", OPT_DKEY, '<',