Allow disabling RPC server TLS for localhost only.

[davecgh]

This pull request introduces a new flag, --notls, which can be used to disable TLS for the RPC server. However, the flag can only be used when the RPC server is bound to localhost interfaces. This is intended to prevent the situation where someone decides they want to expose the RPC server to the web for remote management/access, but forgot they have TLS disabled.

Here is an example of the functionality:

Disallowed

$ ./btcd --notls --rpclisten=whateverdomain.com
loadConfig: the --notls option may not be used when binding to non localhost addresses: whateverdomain.com:8334
Use btcd -h to show usage

$ ./btcd --notls --rpclisten=whateverdomain.com:12345
loadConfig: the --notls option may not be used when binding to non localhost addresses: whateverdomain.com:12345
Use btcd -h to show usage

$ ./btcd --notls --rpclisten=1.2.3.4
loadConfig: the --notls option may not be used when binding to non localhost addresses: 1.2.3.4:8334
Use btcd -h to show usage

$ ./btcd --notls --rpclisten=1.2.3.4:12345
loadConfig: the --notls option may not be used when binding to non localhost addresses: 1.2.3.4:12345
Use btcd -h to show usage

$ ./btcd --notls --rpclisten=1234::1234
loadConfig: the --notls option may not be used when binding to non localhost addresses: [1234::1234]:8334
Use btcd -h to show usage

$ ./btcd --notls --rpclisten=[1234::1234]:12345
loadConfig: the --notls option may not be used when binding to non localhost addresses: [1234::1234]:12345
Use btcd -h to show usage

Allowed

$ ./btcd --notls
17:31:44 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=localhost
17:31:49 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=localhost:12345
17:31:52 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=127.0.0.1
17:32:09 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=127.0.0.1:12345
17:32:13 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=::1
17:32:16 2014-12-20 [INF] BTCD: Version 0.9.0-beta

$ ./btcd --notls --rpclisten=[::1]:12345
17:32:20 2014-12-20 [INF] BTCD: Version 0.9.0-beta

[jcvernaleo]

Change looks good to me. No reason to allow notls when talking to the outside world but there are definitely times where I've need to not use it when local only.