This is a patch for 
[ 514433 ] bsddb: enable dbopen (file==NULL)
from that bug:
"""
bsddb: enable dbopen (file==NULL)
dbopen(): if the file argument is NULL, the library
will use a temporary file. this is useful if you want
that, or if you want to specify a large cache so that
it never actually touches the disk. [i.e., in-memory
hash/bt]
I've done this by replacing the "s" with a "z" in the
arg specs for the three open functions. Seems to work. 
"""

This patch does this. Some testing seems to show that
it works fine.

The docs for db-1.86 show that it was an acceptable
option back then, and it's still allowed for db-3.2,
so that seems a wide enough range of supported 
libraries.

Anyway, this patch has the trivial fix, additions
to the test suite (such as it is) and some short
additions to the docs (lifted from the dbopen()
manpage)

If this is fine, let me know and I'll check it into
the trunk.

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Looks OK. Check it in on the trunk, and we'll see how it
goes. (Could be a 2.2 candidate too I think.)

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checked into the trunk, considering it for 2.2.