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/os-specific/linux/kernel/manual-config.nix

@@ -179,7 +179,7 @@ let
       '' else "") + (if isModular then ''
         mkdir -p $dev
         cp vmlinux $dev/
-        if [ -z "$dontStrip" ]; then
+        if [ -z "''${dontStrip-}" ]; then
           installFlagsArray+=("INSTALL_MOD_STRIP=1")
         fi
         make modules_install $makeFlags "''${makeFlagsArray[@]}" \