[Yunq] Delete client capability in destructor.

Add a method to the server class to create an unowned capability. This
makes it simpler to create a capability for passing to other processes.

Duplicate the init yellowstone cap when using it temporarily.

sys/yellowstone/lib/yellowstone/yellowstone.yunq.server.h

@@ -17,6 +17,7 @@ class YellowstoneServerBase {
   YellowstoneServerBase(YellowstoneServerBase&&) = delete;
   virtual ~YellowstoneServerBase();
 
+  glcr::ErrorOr<z_cap_t> CreateClientCap();
   glcr::ErrorOr<YellowstoneClient> CreateClient();
 
   [[nodiscard]] Thread RunServer();