nghttpx: Add --ecdh-curves option to specify list of named curves

This option requires OpenSSL >= 1.0.2.  With OpenSSL 1.0.2, the
default value is "P-256".  With OpenSSL 1.1.0 or later, the default
value is "X25519:P-256".

gennghttpxfun.py

@@ -146,6 +146,7 @@ OPTIONS = [
     "frontend-http2-decoder-dynamic-table-size",
     "backend-http2-encoder-dynamic-table-size",
     "backend-http2-decoder-dynamic-table-size",
+    "ecdh-curves",
 ]
 
 LOGVARS = [