3c2c439dcd8797019ac6d6614775d5c20ee41c36 wallet: Make -walletdir network only (João Barbosa)
Pull request description:
With this PR `bitcoind -regtest` doesn't run if bitcoin.conf has
```
walletdir=/mnt/mydisk/wallets
```
But works with
```
[regtest]
walletdir=/mnt/mydisk/wallets
```
Doesn't change mainnet behavior.
Closes #15630.
---
Backport of Core PR17447