Revert "berkeleydb: bump to version 6.0.20"

BerkeleyDB has changed its license since version 6. New license (AGPL) may
be incompatible with some packages. So we prefer to stick to version 5

This reverts commit 4c478de2.

[Peter: adjust comment as suggested by Yann]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>