Add a global log policy hook to the logging framework

This addresses the need for a central hook on any log write, which
wasn't previously doable without a lot of effort. The log manager
invokes the new Log::log_stream_policy hook prior to any filter-specific
hooks. Like filter-level hooks, it may veto a log write. Even when
it does, filter-level hooks still get invoked, but cannot "un-veto".

Includes test cases.

src/logging/Manager.h

@@ -299,6 +299,7 @@ private:
 	std::vector<Stream *> streams;	// Indexed by stream enum.
 	int rotations_pending;	// Number of rotations not yet finished.
 	FuncPtr rotation_format_func;
+	FuncPtr log_stream_policy_hook;
 };
 
 } // namespace logging;