nix: Respect -I, --arg, --argstr

Also, random cleanup to argument handling.

src/libexpr/common-eval-args.hh

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "args.hh"
+
+namespace nix {
+
+class Store;
+class EvalState;
+struct Bindings;
+
+struct MixEvalArgs : virtual Args
+{
+    MixEvalArgs();
+
+    Bindings * getAutoArgs(EvalState & state);
+
+    Strings searchPath;
+
+private:
+
+    std::map<std::string, std::string> autoArgs;
+};
+
+Path lookupFileArg(EvalState & state, string s);
+
+}