OpenAlias support

[aguycalled]

This PR completes the implementation of the open alias protocol in the NavCoin Core client.

OpenAlias is an standard created by the Monero Core project used in the Monero wallet and some other clients like Bitcoin Electrum which allows to translate email addresses into cryptocurrency addresses using custom TXT entries in the DNS records of a domain.

More details about the specification can be found at www.openalias.org

Part of the code is based in the original Monero implementation. Main differences are:

• Prefix of the TXT entry is required to be set to oa1:nav
• The only parsed parameter is recipient_address
• NavCoin enforces by default the use of DNSSEC
• Originally Unbound was used to resolve the entry. This created issues when linking the library in some systems, so alternatively support for using Google's DNS over HTTPS service has been implemented. Future implementation of libunbound+dnscrypt is highly recommended to reduce dependence on 3rd parties

Other modifications in the NavCoin client:

• New RPC command resolveopenalias resolves an OpenAlias address to a NavCoin address
• Added support for sending to OpenAlias addresses in the GUI, when parsing URIs and the RPC commands validateaddress and sendtoaddress
• New argument -requirednssec to set whether DNSSEC validation is required to resolve openalias addresses (true by default).
• Update copyright notice

[red010b37]

Don't want to add any more to v 4.2.0-rc, this should be its own minor release

[red010b37]

what does HAVE_UNBOUND relate to in this context?

[aguycalled]

if the library unbound is present we will use it to resolve the dns entry instead of dns over https

[red010b37]

Should we have a test for this?

[red010b37]

There seem to be a lot of ways to get addresses. Is this correct? as we seem to be just calling https://dns.google.com/resolve

What is the common path through here?

[red010b37]

or are we just returning an empty vector on error?

[aguycalled]

exactly, an empty vector is returned when no result is found (whatever the reason for this is)

[red010b37]

and a test for this as double for loops can be a pain