Add (and use) generic reference counter, to improve signal:noise ratio

in code defining reference-counted objects.

src/include/gpxe/refcnt.h

@@ -0,0 +1,39 @@
+#ifndef _GPXE_REFCNT_H
+#define _GPXE_REFCNT_H
+
+/** @file
+ *
+ * Reference counting
+ *
+ */
+
+/**
+ * A reference counter
+ *
+ * This data structure is designed to be embedded within a
+ * reference-counted object.
+ */
+struct refcnt {
+	/** Current reference count
+	 *
+	 * When this count is decremented below zero, the free()
+	 * method will be called.
+	 */
+	int refcnt;
+	/** Free containing object
+	 *
+	 * This method is called when the reference count is
+	 * decremented below zero.
+	 *
+	 * If this method is left NULL, the standard library free()
+	 * function will be called.  The upshot of this is that you
+	 * may omit the free() method if the @c refcnt object is the
+	 * first element of your reference-counted struct.
+	 */
+	void ( * free ) ( struct refcnt *refcnt );
+};
+
+extern void ref_get ( struct refcnt *refcnt );
+extern void ref_put ( struct refcnt *refcnt );
+
+#endif /* _GPXE_REFCNT_H */