Omit User-Agent: header by default

Omit User-Agent: header by default

The User-Agent: header can be fun and interesting, 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).

This change also avoids hiding the User-Agent header by default, so
that people who decide they want to send it will at least see it (and
can edit it if they want to) before sending.

It makes sense to have safer defaults.