nix: Make all options available as flags

Thus, instead of ‘--option <name> <value>’, you can write ‘--<name>
<value>’. So

  --option http-connections 100

becomes

  --http-connections 100

Apart from brevity, the difference is that it's not an error to set a
non-existent option via --option, but unrecognized arguments are
fatal.

Boolean options have special treatment: they're mapped to the
argument-less flags ‘--<name>’ and ‘--no-<name>’. E.g.

  --option auto-optimise-store false

becomes

  --no-auto-optimise-store

src/libutil/config.hh

@@ -63,6 +63,8 @@ public:
     void resetOverriden();
 
     void toJSON(JSONObject & out);
+
+    void convertToArgs(Args & args);
 };
 
 class AbstractSetting
@@ -99,6 +101,8 @@ protected:
 
     virtual void toJSON(JSONPlaceholder & out);
 
+    virtual void convertToArg(Args & args);
+
     bool isOverriden() { return overriden; }
 };
 
@@ -132,6 +136,8 @@ public:
 
     std::string to_string() override;
 
+    void convertToArg(Args & args) override;
+
     void toJSON(JSONPlaceholder & out) override;
 };