Expand several more config options.
Expand several more config options.

This adds home directory and OS environment variable expansion for the
following configuration options: configfile, appdatadir, rpccert,
rpckey.

synrg-labs/btcwallet,Stash-Crypto/btcwallet,btcsuite/btcwallet,jrick/dcrwallet,synrg-labs/btcwallet,Roasbeef/btcwallet,Roasbeef/btcwallet,decred/dcrwallet,btcsuite/btcwallet,jrick/dcrwallet,Stash-Crypto/btcwallet,decred/dcrwallet,jrick/btcwallet,jrick/btcwallet