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