introduce and use the generalized node::Warnings interface

Instead of having separate warning functions (and globals) for each
different warning that can be raised, encapsulate this logic into
a single class and allow to (un)set any number of warnings.

Introduces behaviour change:
- the `-alertnotify` command is executed for all
  `KernelNotifications::warningSet` calls, which now also covers the
  `LARGE_WORK_INVALID_CHAIN` warning.
- previously, warnings were returned based on a predetermined order,
  e.g. with the "pre-release test build" warning always first. This
  is no longer the case, and Warnings::GetMessages() will return
  messages sorted by the id of the warning.

Removes warnings.cpp from kernel.

src/node/interfaces.cpp

@@ -93,7 +93,7 @@ public:
     explicit NodeImpl(NodeContext& context) { setContext(&context); }
     void initLogging() override { InitLogging(args()); }
     void initParameterInteraction() override { InitParameterInteraction(args()); }
-    bilingual_str getWarnings() override { return Join(GetWarnings(), Untranslated("<hr />")); }
+    bilingual_str getWarnings() override { return Join(node::g_warnings.GetMessages(), Untranslated("<hr />")); }
     int getExitStatus() override { return Assert(m_context)->exit_status.load(); }
     uint32_t getLogCategories() override { return LogInstance().GetCategoryMask(); }
     bool baseInitialize() override