context: for dhparam_cb, pass is_export as boolean

The integer value that's actually returned for this flag is 2, which is fine
for C (it is defined as true), but it's sufficiently surprising (because it's
not 1), that this is worth fixing -- even if export ciphers aren't common.

It should be a boolean anyway.

src/context.c

@@ -167,7 +167,7 @@ static DH *dhparam_cb(SSL *ssl, int is_export, int keylength)
   lua_gettable(L, -2);
 
   /* Invoke the callback */
-  lua_pushnumber(L, is_export);
+  lua_pushboolean(L, is_export);
   lua_pushnumber(L, keylength);
   lua_call(L, 2, 1);