Omit User-Agent: header by default

Omit User-Agent: header by default

The User-Agent: header can be fun and interesting and useful for
debugging, but it also leaks quite a bit of information about the user
and their software stack.

This represents a potential security risk (attackers can target the
particular stack) and also an anonymity risk (a user trying to
preserve their anonymity by sending mail from a non-associated account
might reveal quite a lot of information if their choice of mail user
agent is exposed).

Users who want to configure `user_agent` to `yes` can still do so, but
it makes sense to have safer defaults.

Co-authored-by: Richard Russon <rich@flatcap.org>