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.

Add C99-required 1st parameter.  Disable for for non-gcc.  Simplify.
GNU extension #define DPRINTF(<no 1st arg>...) -> GNU ext ,##__VA_ARGS__.
Fix __LINE__ format %ld -> %d.