This commit factors out the btcd and ltcd options into their own sections
similar to neutrino, and adds a bitcoind section as well. Now, you specify
node options similarly to:
--ltcd.rpchost=...
or
--btcd.rpcuser=...
or
--bitcoind.zmqpath=...
For Bitcoin, you specify an alternate back-end to btcd as follows:
--bitcoin.node=bitcoind
or
--bitcoin.node=neutrino
You can also specify the default option:
--bitcoin.node=btcd
For Litecoin, only `btcd` mode is valid, and corresponds to the `ltcd`
section. For example:
--litecoin.node=btcd
--ltcd.rpchost=...
The new code also attempts to read the correct options and auth info
from bitcoin.conf just as it does from btcd.conf/ltcd.conf.
