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-build/nix-build.cc

@@ -232,6 +232,8 @@ void mainWrapped(int argc, char * * argv)
 
     myArgs.parseCmdline(args);
 
+    initPlugins();
+
     if (packages && fromArgs)
         throw UsageError("'-p' and '-E' are mutually exclusive");