MINOR: pools: distinguish the requested alignment from the type-specific one

MINOR: pools: distinguish the requested alignment from the type-specific one

We're letting users request an alignment but that can violate one imposed
by a type, especially if we start seeing REGISTER_TYPED_POOL() grow in
adoption, encouraging users to specify alignment on their types. On the
other hand, if we ask the user to always specify the alignment, no control
is possible and the error is easy. Let's have a second field in the pool
registration, for the type-specific one. We'll set it to zero when unknown,
and to the types's alignment when known. This way it will become possible
to compare them at startup time to detect conflicts. For now no macro
permits to set both separately so this is not visible.