chasquid needs at least one certificate in order to start, to prevent
accidental misconfigurations.
This patch documents this in etc/chasquid/README, and while at it fixes
a minor terminology issue.
Thanks to Martin Ferrari for the bug report!
Picking the domain used in the DSN message "From" header is more
complicated than it needs to be, causing confusing code paths and having
different uses for the hostname, which should be purely aesthetic.
This patch makes the queue pick the DSN "From" domain from the message
itself, by looking for a local domain in either the sender or the
original recipients. We should find at least one, otherwise it'd be
relaying.
This allows the code to be simplified, and we can narrow the scope of
the hostname option even further.
