smptd: ignore broken pipe errors from child
smptd: ignore broken pipe errors from child

cmd.Run might return an error even if the process finished successfully,
for example when running a process command which succeeds before reading
the entire email. This is supported, and we use that case in our tests.
(whether or not that makes sense is a different matter.)

kinkome/kinko.smtpd,kinkome/kinko.smtpd,kinkome/kinko.smtpd