Note derived from Martin Hedenfalk's btree.c; the code
is significantly reworked but owes a lot to the original.

Fixed order of mdb_del0 operations
Drop in-memory free list from env on txn_abort

For compat with ancient BSD. Will probably just copy <ldap_queue.h> instead.

Go back to using an mmap'd lock file

it's useless, just use meta pages instead

NUMKEYS was size_t due to PAGEHDRSZ; printing unsigned int is simpler.

Instead increase PAGEFILL to long, since it involves a multiply.
Possibly uint32_t or something would be sufficient.