fd: make the close method on IOHandle private

Fd's cache does not handle changes in the state of its IOHandle parameter.
If we allow:

	Fd f;
	f->close();

then Fd ends up caching a pointer to a closed Fd, and will become very
badly confused if a new Fd appears with the same int identifier.

Fix by removing the close method.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/fd.h

@@ -35,11 +35,11 @@ namespace crucible {
 		IOHandle& operator=(IOHandle &&) = delete;
 		IOHandle& operator=(const IOHandle &) = delete;
 		int	m_fd;
+		void close();
 	public:
 		virtual ~IOHandle();
 		IOHandle(int fd = -1);
 		int get_fd() const;
-		void close();
 	};
 
 	class Fd {