Propose to MDB_PICKUP for mdb_dpi_open()

[Леонид Юрьев <leo@yuriev.ru>]

Hi, Howard.

I would like to introduce the MDB_PICKUP flag for mdb_dpi_open().

Please see https://github.com/ReOpen/ReOpenLDAP/commit/04f92c952e42ff4c843135c42fdb9aa4b70a34fe

At least, this is required for mdb_chk tool, when a DBI should be opened with flags that stored inside LMDB, which while a DBI was created. It is particularly important for DBI_MAIN.

Leonid.