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-daemon/nix-daemon.cc

@@ -1060,6 +1060,8 @@ int main(int argc, char * * argv)
             return true;
         });
 
+        initPlugins();
+
         if (stdio) {
             if (getStoreType() == tDaemon) {
                 /* Forward on this connection to the real daemon */