rpcclient: Variable coin symbol

[davecgh]

From @goarchit on February 4, 2017 3:21

Currently client.ListAccounts() returns a balance amount with the symbol "BTC" hard coded in as a suffix.

Please add the ability to substitute in a different symbol, perhaps as a field in chaincfg.Params, in order to add additional flexibility in supporting other cybercoins.

Copied from original issue: btcsuite/btcrpcclient#118