Enable TLS support for btcd websocket connections.

This adds an additional config option, -cafile, to specify the root
certificates checked when verifying a btcd TLC connection.  btcd will
now automatically generate certs in
~/.btcd/data/{main,test}net/rpc.cert, and this file should be copied
to ~/.btcwallet/cert.pem.

The -btcdport option is also gone now, and replaced with -connect (or
-c), to specify both the hostname/ip and port of the server running
btcd.