MEDIUM: pools: add an alignment property

This will be used to declare aligned pools. For now it's not used,
but it's properly set from the various registrations that compose
a pool, and rounded up to the next power of 2, with a minimum of
sizeof(void*).

The alignment is returned in the "show pools" part that indicates
the entry size. E.g. "(56 bytes/8)" means 56 bytes, aligned by 8.

include/haproxy/pool-t.h

@@ -128,6 +128,7 @@ struct pool_head {
 	unsigned int minavail;	/* how many chunks are expected to be used */
 	unsigned int size;	/* chunk size */
 	unsigned int flags;	/* MEM_F_* */
+	unsigned int align;     /* alignment size */
 	unsigned int users;	/* number of pools sharing this zone */
 	unsigned int alloc_sz;	/* allocated size (includes hidden fields) */
 	unsigned int sum_size;	/* sum of all registered users' size */