config: Add bitcoind.{config,rpccookie} configuration options

Currently, the Bitcoind.Dir configuration option is used as the base
directory for locating both the bitcoind configuration file and the RPC
cookie file. However, it is quite common for Bitcoin Core to be packaged
in such a way that the configuration file and the RPC cookie file reside
in different directories: "/etc/bitcoin/bitcoin.conf" and
"/var/lib/bitcoind/.cookie".

This change makes it such that --bitcoind.config and
--bitcoind.rpccookie options can be specified to override the default
auto-detection logic, and if either is unspecified, the auto-detection
logic will still do its job.

sample-lnd.conf

@@ -578,6 +578,12 @@ bitcoin.node=btcd
 ; etc.
 ; bitcoind.dir=~/.bitcoin
 
+; Configuration filepath.
+; bitcoind.config=~/.bitcoin/bitcoin.conf
+
+; Authentication cookie file for RPC connections.
+; bitcoind.rpccookie=~/.bitcoin/.cookie
+
 ; The host that your local bitcoind daemon is listening on. By default, this
 ; setting is assumed to be localhost with the default port for the current
 ; network.
@@ -779,6 +785,12 @@ litecoin.node=ltcd
 ; etc.
 ; litecoind.dir=~/.litecoin
 
+; Configuration filepath.
+; litecoind.config=~/.litecoin/litecoin.conf
+
+; Authentication cookie file for RPC connections.
+; litecoind.rpccookie=~/.litecoin/.cookie
+
 ; The host that your local litecoind daemon is listening on. By default, this
 ; setting is assumed to be localhost with the default port for the current
 ; network.