Revise error codes

[anyuta1166]

Revise error codes and add human-readable descriptions (for example, RC_DYNDNS_RSP_NOTOK - "Error in DDNS server response").

[troglobit]

Fixed, or at the very least greatly improved upstream. See troglobit/inadyn@7e62411

[anyuta1166]

Oh, necroposting. This fork was created years ago for usage in the custom router firmware and is not maintained any more, as well as the firmware itself.

[troglobit]

@vampik Ah, good to know. The issues posted here have been a great help though. Also, in case you're interested, I've merged most (all I think) patches in your fork to upstream. Thanks!