Changes from the earlier BETA releases.

o dbm_prep does everything now, so dbm_open is just a simple
  wrapper that builds the default filenames. dbm_prep no longer
  requires a (DBM *) db parameter: it allocates one itself. It
  returns (DBM *) db or (DBM *) NULL.

o makroom is now reliable. In the common-case optimization of the page
  split, the page into which the incoming key/value pair is to be inserted
  is write-deferred (if the split is successful), thereby saving a cosly
  write.  BUT, if the split does not make enough room (unsuccessful), the
  deferred page is written out, as the failure-window is now dependent on
  the number of split attempts.

o if -DDUFF is defined, hash function will also use the DUFF construct.
  This may look like a micro-performance tweak (maybe it is), but in fact,
  the hash function is the third most-heavily used function, after read
  and write.