Add plugins to make Nix more extensible.

All plugins in plugin-files will be dlopened, allowing them to
statically construct instances of the various Register* types Nix
supports.

src/nix-copy-closure/nix-copy-closure.cc

@@ -44,6 +44,8 @@ int main(int argc, char ** argv)
             return true;
         });
 
+        initPlugins();
+
         if (sshHost.empty())
             throw UsageError("no host name specified");