Use smarter btcd cert path logic.

If ~/.btcwallet/btcd.cert does not exist and the CA file has not been
explicitly set using the config file or command line flags, it's
possible that the cert can be found in ~/.btcd.  If connecting to a
localhost btcd and the previous statements are true, the default CA
file config option is updated for the certificate in them btcd
homedir.

If ~/.btcwallet/btcd.cert does exist and the CA file has not been set,
it is used without checking for a cert in the btcd homedir.