Modify btcctl to accept --testnet and --wallet flags

[davecgh]

Rather than expecting users to figure out which port and certs to use, it would be easier if btcctl accepted --testnet and --wallet flags which set the defaults accordingly.

In particular the following chart shows the port that should be selected for each of the combinations of the two flags:

	Without --wallet	With --wallet
Without --testnet	8334	8332
With --testnet	18334	18332

These flags should be ignored if a specific port is specified via -s/--rpcserver, however if no port is specified via that flag, the default port per the above chart should be appended.

This will allow syntax such as btcctl --testnet -s 192.168.x.x in addition to btcctl -s 192.168.x.x:1234.

Also, the --wallet flag should set the default certs to the wallet certs if they're on the local filesystem (in the same fashion as the current code finds the btcd certs).