Splits Modules/_bsddb.c up into bsddb.h and _bsddb.c and adds a C API

Splits Modules/_bsddb.c up into bsddb.h and _bsddb.c and adds a C API
object available as bsddb.db.api.  This is based on the patch submitted
by Duncan Grisby here:
  http://sourceforge.net/tracker/index.php?func=detail&aid=1551895&group_id=13900&atid=313900
See this thread for additional info:
  http://sourceforge.net/mailarchive/forum.php?thread_name=E1GAVDK-0002rk-Iw%40apasphere.com&forum_name=pybsddb-users

It also cleans up the code a little by removing some ifdef/endifs for
python prior to 2.1 and for unsupported Berkeley DB <= 3.2.