treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/development/python-modules/pythonnet/default.nix

@@ -43,8 +43,8 @@ buildPythonPackage rec {
   '';
 
   preConfigure = ''
-    [ -z "$dontPlacateNuget" ] && placate-nuget.sh
-    [ -z "$dontPlacatePaket" ] && placate-paket.sh
+    [ -z "''${dontPlacateNuget-}" ] && placate-nuget.sh
+    [ -z "''${dontPlacatePaket-}" ] && placate-paket.sh
   '';
 
   nativeBuildInputs = [