Add encrypted/authenticated IPC listener support

[jagerman]

Something that would be very useful is to be able to expose an encrypted/authenticated but public listener. This needs two config additions:

1. We need to be able to specify the address on which we listen_curve().
2. We need to be able to specify one (or more) public keys that will be accepted.

Something like this in the config:

[api]
bind_curve = tcp://0.0.0.0:1234
curve_pubkey = abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789

This would then use listen_curve() on that address (in addition to any of the listen_plain()s that happen from the bind options currently).

listen_curve() takes an "allow" callback: the allow callback would be looking at the public key and return AuthLevel::admin if it is in the curve_pubkey list, otherwise AuthLevel::denied to refuse the connection.

Originally posted by @jagerman in #2111 (comment)