Set default BDB version in bitcoin slave ports

In c465619 a new single-choice option was introduced in net-p2p/bitcoin
where exactly one variant must be chosen (BDB version). But the bitcoin
slave ports net-p2p/bitcoin-daemon and net-p2p/bitcoin-utils set
OPTIONS_DEFAULT=... (with missing BDB version) and the master port uses
OPTIONS_DEFAULT?=...somebdbversion... so the choice of the master port
is not inherited by the slaves.

Fix this by explicitly setting the BDB version in both slave ports.

Reported by: Dan Langille