These ops could fail if the map is full.

Don't try to reclaim overflow pages while operating on
the freelist (for now). The circular dependencies are much like
the single-page case, but worse. Maybe look into this in the
future, but it's not absolutely necessary now.

Keep list sorted if it grows during a write. Don't free pghead until
we're sure our writes are all finished.

Just write in 2^31 sized chunks

Avoid possible symbol clashes with MDB-Tools libmdb

mdb_cursor_sibling() no longer pops cursor before returning.

liblmdb

Avoid naming conflicts with other mdb* packages. Bump version to 0.9.5

With MDB_MAPASYNC, the API provided no way to ensure full sync.

It returned 0, which is the freelist DBI and is valid for a few uses.

This allows for other flag values in the future.

Let the user specify the number of databases he will actually
use. Adding 2 for FREE_DBI and MAIN_DBI is an internal matter.

mdb_env_open() with these flags did not open a synchronizing
file descriptor.  Thus disabling them later did not work.