config.go: check for RPCUser AND RPCPass (AND ZMQPath for bitcoind)
config.go: check for RPCUser AND RPCPass (AND ZMQPath for bitcoind)

This commit causes the configuration parser to accept the values of
RPCUser, RPCPass, and (for the bitcoind back-end) ZMQPath configured
in lnd.conf or on the command line ONLY when all are specified. It
causes the configuration parser to look in btcd.conf or bitcoin.conf
ONLY when none are specified, and causes an error to be returned when
only some are specified, as users have done so erroneously and the
lack of clear feedback has caused difficulties.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd