MEDIUM: pools: add an alignment property

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.