nixos/nixos-option: Rewrite in a more suitable language

Also add --all, which shows the value of all options.  Diffing the --all
output on either side of contemplated changes is a lovely way to better
understand what's going on inside nixos.

nixos/modules/installer/tools/nixos-option/libnix-copy-paste.hh

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <iostream>
+#include <nix/types.hh>
+#include <string>
+
+nix::Strings parseAttrPath(const std::string &s);
+bool isVarName(const std::string &s);
+std::ostream &printStringValue(std::ostream &str, const char *string);