This directory provides a wrapper using the ydb db.h header file but
it calls the bdb internals.  To get this to work requires a little
magic, since both BDB and YDB use the same type names. 

BDB helps with a ./configure option (--with-uniquename) that makes it
so that all the link-time symbols have different names.  But the type
names and enums collide.  Hence we cannot include both the
../include/db.h and the BDB db.h.

To fix that we have a header ydb-uniq.h, which uses completely
different names for everything in the ydb interface.  But those names
are compatible with the standard names: That is all the structs have
the same layout.

Thus we have the following three headers
 bdb_db.h  which defines things like db_env_create_4001(DBENV **, ...)

 ydb_db.h  which defines things like db_env_create(DBENV **, ...)       (but the DBENV is a different type from BDB's)
 bdbw.h which defines things like db_env_create_ydb(DBENV_ydb **, ...)

bdbw.h can be included with bdb_db.h or ydb_db.h
bdb_db.h and ydb_db.h cannot both be included in the same file.

An application, such as mysql, includes db.h from ../include
That is the ydb header file.