Log full paths for wallets (Hennadii Stepanov)
Pull request description:
Fix #15333
`debug.log` with this PR:
```
...
2019-02-03T19:02:35Z Using wallet directory /home/hebasto/.bitcoin/testnet3/wallets
2019-02-03T19:02:35Z init message: Verifying wallet(s)...
2019-02-03T19:02:35Z Using BerkeleyDB version Berkeley DB 4.8.30: (April 9, 2010)
2019-02-03T19:02:35Z Using wallet test_alpha/wallet.dat
2019-02-03T19:02:35Z BerkeleyEnvironment::Open: LogDir=/home/hebasto/.bitcoin/testnet3/wallets/test_alpha/database ErrorFile=/home/hebasto/.bitcoin/testnet3/wallets/test_alpha/db.log
2019-02-03T19:02:35Z Using BerkeleyDB version Berkeley DB 4.8.30: (April 9, 2010)
2019-02-03T19:02:35Z Using wallet alpha_wallet/wallet.dat
2019-02-03T19:02:35Z BerkeleyEnvironment::Open: LogDir=/home/hebasto/.bitcoin/testnet3/wallets/alpha_wallet/database ErrorFile=/home/hebasto/.bitcoin/testnet3/wallets/alpha_wallet/db.log
2019-02-03T19:02:35Z Using BerkeleyDB version Berkeley DB 4.8.30: (April 9, 2010)
2019-02-03T19:02:35Z Using wallet wallet.dat
2019-02-03T19:02:35Z BerkeleyEnvironment::Open: LogDir=/home/hebasto/.bitcoin/testnet3/wallets/database ErrorFile=/home/hebasto/.bitcoin/testnet3/wallets/db.log
2019-02-03T19:02:35Z Using BerkeleyDB version Berkeley DB 4.8.30: (April 9, 2010)
2019-02-03T19:02:35Z Using wallet none/wallet.dat
2019-02-03T19:02:35Z BerkeleyEnvironment::Open: LogDir=/home/hebasto/.bitcoin/testnet3/wallets/none/database ErrorFile=/home/hebasto/.bitcoin/testnet3/wallets/none/db.log
2019-02-03T19:02:35Z init message: Loading banlist...
...
```
https://github.com/bitcoin/bitcoin/pull/15334/commits/a4b92e467dd182621497deda1e80a9737629c75f
---
Backport of Core PR15334