Add new FOREACH_FROM variants for queue(3)

Add new FOREACH_FROM variants of the queue(3) FOREACH macros which can
optionally start the traversal from a previously found element by
passing the element in as "var". Passing a NULL "var" retains the same
semantics as the regular FOREACH macros.

Kudos to phk for suggesting the "FROM" suffix instead of my original
proposal.

Reviewed by:	jhb (previous version), rpaulo
MFC after:	1 week

newlib/libc/include/sys/queue.h

@@ -88,9 +88,13 @@
  * _PREV			-	+	-	+
  * _LAST			-	-	+	+
  * _FOREACH			+	+	+	+
+ * _FOREACH_FROM		+	+	+	+
  * _FOREACH_SAFE		+	+	+	+
+ * _FOREACH_FROM_SAFE		+	+	+	+
  * _FOREACH_REVERSE		-	-	-	+
+ * _FOREACH_REVERSE_FROM	-	-	-	+
  * _FOREACH_REVERSE_SAFE	-	-	-	+
+ * _FOREACH_REVERSE_FROM_SAFE	-	-	-	+
  * _INSERT_HEAD			+	+	+	+
  * _INSERT_BEFORE		-	+	-	+
  * _INSERT_AFTER		+	+	+	+
@@ -167,11 +171,21 @@ struct {								\
 	    (var);							\
 	    (var) = SLIST_NEXT((var), field))
 
+#define	SLIST_FOREACH_FROM(var, head, field)				\
+	for ((var) = ((var) ? (var) : SLIST_FIRST((head)));		\
+	    (var);							\
+	    (var) = SLIST_NEXT((var), field))
+
 #define	SLIST_FOREACH_SAFE(var, head, field, tvar)			\
 	for ((var) = SLIST_FIRST((head));				\
 	    (var) && ((tvar) = SLIST_NEXT((var), field), 1);		\
 	    (var) = (tvar))
 
+#define	SLIST_FOREACH_FROM_SAFE(var, head, field, tvar)			\
+	for ((var) = ((var) ? (var) : SLIST_FIRST((head)));		\
+	    (var) && ((tvar) = SLIST_NEXT((var), field), 1);		\
+	    (var) = (tvar))
+
 #define	SLIST_FOREACH_PREVPTR(var, varp, head, field)			\
 	for ((varp) = &SLIST_FIRST((head));				\
 	    ((var) = *(varp)) != NULL;					\
@@ -259,12 +273,21 @@ struct {								\
 	   (var);							\
 	   (var) = STAILQ_NEXT((var), field))
 
+#define	STAILQ_FOREACH_FROM(var, head, field)				\
+	for ((var) = ((var) ? (var) : STAILQ_FIRST((head)));		\
+	   (var);							\
+	   (var) = STAILQ_NEXT((var), field))
 
 #define	STAILQ_FOREACH_SAFE(var, head, field, tvar)			\
 	for ((var) = STAILQ_FIRST((head));				\
 	    (var) && ((tvar) = STAILQ_NEXT((var), field), 1);		\
 	    (var) = (tvar))
 
+#define	STAILQ_FOREACH_FROM_SAFE(var, head, field, tvar)		\
+	for ((var) = ((var) ? (var) : STAILQ_FIRST((head)));		\
+	    (var) && ((tvar) = STAILQ_NEXT((var), field), 1);		\
+	    (var) = (tvar))
+
 #define	STAILQ_INIT(head) do {						\
 	STAILQ_FIRST((head)) = NULL;					\
 	(head)->stqh_last = &STAILQ_FIRST((head));			\
@@ -394,11 +417,21 @@ struct {								\
 	    (var);							\
 	    (var) = LIST_NEXT((var), field))
 
+#define	LIST_FOREACH_FROM(var, head, field)				\
+	for ((var) = ((var) ? (var) : LIST_FIRST((head)));		\
+	    (var);							\
+	    (var) = LIST_NEXT((var), field))
+
 #define	LIST_FOREACH_SAFE(var, head, field, tvar)			\
 	for ((var) = LIST_FIRST((head));				\
 	    (var) && ((tvar) = LIST_NEXT((var), field), 1);		\
 	    (var) = (tvar))
 
+#define	LIST_FOREACH_FROM_SAFE(var, head, field, tvar)			\
+	for ((var) = ((var) ? (var) : LIST_FIRST((head)));		\
+	    (var) && ((tvar) = LIST_NEXT((var), field), 1);		\
+	    (var) = (tvar))
+
 #define	LIST_INIT(head) do {						\
 	LIST_FIRST((head)) = NULL;					\
 } while (0)
@@ -531,21 +564,41 @@ struct {								\
 	    (var);							\
 	    (var) = TAILQ_NEXT((var), field))
 
+#define	TAILQ_FOREACH_FROM(var, head, field)				\
+	for ((var) = ((var) ? (var) : TAILQ_FIRST((head)));		\
+	    (var);							\
+	    (var) = TAILQ_NEXT((var), field))
+
 #define	TAILQ_FOREACH_SAFE(var, head, field, tvar)			\
 	for ((var) = TAILQ_FIRST((head));				\
 	    (var) && ((tvar) = TAILQ_NEXT((var), field), 1);		\
 	    (var) = (tvar))
 
+#define	TAILQ_FOREACH_FROM_SAFE(var, head, field, tvar)			\
+	for ((var) = ((var) ? (var) : TAILQ_FIRST((head)));		\
+	    (var) && ((tvar) = TAILQ_NEXT((var), field), 1);		\
+	    (var) = (tvar))
+
 #define	TAILQ_FOREACH_REVERSE(var, head, headname, field)		\
 	for ((var) = TAILQ_LAST((head), headname);			\
 	    (var);							\
 	    (var) = TAILQ_PREV((var), headname, field))
 
+#define	TAILQ_FOREACH_REVERSE_FROM(var, head, headname, field)		\
+	for ((var) = ((var) ? (var) : TAILQ_LAST((head), headname));	\
+	    (var);							\
+	    (var) = TAILQ_PREV((var), headname, field))
+
 #define	TAILQ_FOREACH_REVERSE_SAFE(var, head, headname, field, tvar)	\
 	for ((var) = TAILQ_LAST((head), headname);			\
 	    (var) && ((tvar) = TAILQ_PREV((var), headname, field), 1);	\
 	    (var) = (tvar))
 
+#define	TAILQ_FOREACH_REVERSE_FROM_SAFE(var, head, headname, field, tvar) \
+	for ((var) = ((var) ? (var) : TAILQ_LAST((head), headname));	\
+	    (var) && ((tvar) = TAILQ_PREV((var), headname, field), 1);	\
+	    (var) = (tvar))
+
 #define	TAILQ_INIT(head) do {						\
 	TAILQ_FIRST((head)) = NULL;					\
 	(head)->tqh_last = &TAILQ_FIRST((head));			\