Rename FROMFIELD to 'container_of'.

Ian Jackson points out that the Linux kernel has a macro of this name
with the same purpose, and suggests that it's a good idea to use the
same name as they do, so that at least some people reading one code
base might recognise it from the other.

I never really thought very hard about what order FROMFIELD's
parameters should go in, and therefore I'm pleasantly surprised to
find that my order agrees with the kernel's, so I don't have to
permute every call site as part of making this change :-)

defs.h

@@ -97,7 +97,7 @@ typedef struct PacketProtocolLayer PacketProtocolLayer;
 
 /* Return a pointer to the object of structure type 'type' whose field
  * with name 'field' is pointed at by 'object'. */
-#define FROMFIELD(object, type, field)                                  \
+#define container_of(object, type, field)                               \
     TYPECHECK(object == &((type *)0)->field,                            \
               ((type *)(((char *)(object)) - offsetof(type, field))))