Original issue: namecoin/namecoin-core#148.

Currently the rpcpassword must be specified in the configuration file. On some systems (for example in NixOS) configuration files are stored in world-readable locations. It's a bad idea to store secrets in world-readable locations.

To better support these systems it would be nice if there was a rpcpasswordFile configuration parameter. Users can then keep the configuration file in a world-readable location but set rpcpasswordFile to a file with restricted ownership and permissions.