dataman: add dm_trylock & add lock for FENCE_POINTS items

src/modules/dataman/dataman.h

@@ -127,16 +127,30 @@ dm_write(
 	size_t buflen			/* Length in bytes of data to retrieve */
 );
 
-/** Lock all items of this type */
-__EXPORT void
+/**
+ * Lock all items of a type. Can be used for atomic updates of multiple items (single items are always updated
+ * atomically).
+ * Note that this lock is independent from dm_read & dm_write calls.
+ * @return 0 on success and lock taken, -1 on error (lock not taken, errno set)
+ */
+__EXPORT int
 dm_lock(
-	dm_item_t item			/* The item type to clear */
+	dm_item_t item			/* The item type to lock */
 );
 
-/** Unlock all items of this type */
+/**
+ * Try to lock all items of a type (@see sem_trywait()).
+ * @return 0 if lock is taken, -1 otherwise (on error or if already locked. errno is set accordingly)
+ */
+__EXPORT int
+dm_trylock(
+	dm_item_t item			/* The item type to lock */
+);
+
+/** Unlock all items of a type */
 __EXPORT void
 dm_unlock(
-	dm_item_t item			/* The item type to clear */
+	dm_item_t item			/* The item type to unlock */
 );
 
 /** Erase all items of this type */