add pool_walk as debug code.
add pool_walk as debug code.

this can be used to walk over all the items allocated with a pool and have
them examined by a function the caller provides.

with help from and ok tedu@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars