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/build-remote/build-remote.cc

@@ -64,6 +64,8 @@ int main (int argc, char * * argv)
 
         settings.maxBuildJobs.set("1"); // hack to make tests with local?root= work
 
+        initPlugins();
+
         auto store = openStore().cast<LocalStore>();
 
         /* It would be more appropriate to use $XDG_RUNTIME_DIR, since