Make LHASH_OF() and STACK_OF() use opaque structs

This removes internals of these two special snowflakes and will allow
further simplifications. Unfortunately, there are some pieces of
software that actually use LHASH_OF() (looking at you, pound, Ruby, and
openssl(1)), so we get to keep exposing this garbage, at least for now.

Expose lh_error() as a symbol to replace a macro reaching into _LHASH.
lh_down_load() is no longer available. _LHASH and _STACK are now opaque,
LHASH_NODE becomes internal-only.

from jsing