Make default config relative to appdata directory.
Make default config relative to appdata directory.

This modifies the default configuration file to be relative to the
application data directory (configuratble with --appdata).  If there
is no configuration file in this directory, then no additional
configuration options are applied (it does not fallback to
~/.btcwallet/btcwallet.conf).

Fixes #421.

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