Machine hostname is exposed to the server

[link2xt]

HELO command sends ClientId::hostname() result which exposes machine hostname to the server.

Some server add it to Received header: https://support.delta.chat/t/delta-chat-disroot-expose-ip-and-machine-name-on-headers/1081

A way to override it is needed, and maybe a better default, such as localhost always.

[gerryfrancis]

maybe a better default, such as localhost always

@link2xt localhost is reserved to IP address 127.0.0.1. Shall the real address be exchanged with the one of localhost?