Allow dnschain.conf to specify path to the namecoin configuration file

[taoeffect]

Currently it looks in HOME/.namecoin/namecoin.conf on linux, and that's annoying because DNSChain runs (via our systemd unit file) as a different user than namecoin runs, and therefore it doesn't see (and doesn't have access to) the namecoin configuration file.

Currently fixing this (when the two are running as different users) is hackish and involves making the group on that file dns and creating a symlink from dns user's home directory into namecoin user's home directory (for the .namecoin folder).

Super hackish, this needs to be fixed, and done for all supported blockchains.

[taoeffect]

Closed in 66324ba.