Use general hardening flag toggle lists

The following parameters are now available:

  * hardeningDisable
    To disable specific hardening flags
  * hardeningEnable
    To enable specific hardening flags

Only the cc-wrapper supports this right now, but these may be reused by
other wrappers, builders or setup hooks.

cc-wrapper supports the following flags:

  * fortify
  * stackprotector
  * pie (disabled by default)
  * pic
  * strictoverflow
  * format
  * relro
  * bindnow

pkgs/servers/http/nginx/default.nix

@@ -55,7 +55,7 @@ stdenv.mkDerivation rec {
 
   preConfigure = concatMapStringsSep "\n" (mod: mod.preConfigure or "") modules;
 
-  hardening_pie = true;
+  hardeningEnable = [ "pie" ];
 
   meta = {
     description = "A reverse proxy and lightweight webserver";