The check whether use the old kwallet backend (ksecretd) or another
Secret Service provider is moved uniquely in SecretServiceClient
as the sole source of truth
The Name UseKSecretBackend is more appropriate than useKWalletBackend

Read isWalletEnabled in kwalletd too: for compatibility with the old service, read "Enabled" in "Wallet"
of kwalletrc as well, to have a completely disabled kwallet state
if someone wants that due to security concerns