`-zapwallettxes` should be disallowed when starting bitcoin in multiwallet mode.

There's code in `WalletInit::ParameterInteraction()` to disallow `-zapwallettxes` when running in multiwallet mode. This code functioned as expected when passing the parameter `-zapwallettxes=1`, but not when passing the parameter `-zapwallettxes` (ie without the value specified). Fix that and add a test.

The new test in the
_[wallet] [tests] Test disallowed multiwallet params_ commit reproduces the bug and should fail against master.