cleanup: rename key and item_head accessors to more friendly names

This patch does a quick search and replace:
B_N_PITEM_HEAD() -> item_head()
B_N_PDELIM_KEY() -> internal_key()
B_N_PKEY() -> leaf_key()
B_N_PITEM() -> item_body()

And the item_head version:
B_I_PITEM() -> ih_item_body()
I_ENTRY_COUNT() -> ih_entry_count()

And the treepath variants:
get_ih() -> tp_item_head()
PATH_PITEM_HEAD() -> tp_item_head()
get_item() -> tp_item_body()

... which makes the code much easier on the eyes.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>

fsck/pass4.c

@@ -28,7 +28,7 @@ void pass_4_check_unaccessed_items(void)
 		if (!fsck_quiet(fs))
 			print_how_fast(items++, 0, 50, 0);
 
-		for (i = get_item_pos(&path), ih = get_ih(&path);
+		for (i = get_item_pos(&path), ih = tp_item_head(&path);
 		     i < B_NR_ITEMS(bh); i++, ih++) {
 			if (!is_item_reachable(ih)) {
 				PATH_LAST_POSITION(&path) = i;