Default walletdir is wallets/ if it exists

Summary:
Backport 2/4 of core PR11466 (commit 9587a9c)

Depends on D2312

Test Plan:

make check
mkdir ~/.bitcoin/wallets
./bitcoind

Check the wallet.dat and db.log files are created in the wallets dir

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2314