lib-smtp: server: Perform output stream error handling in one place.

lib-smtp: server: Use the stream_errno to evaluate output stream errors. Code was using global errno.

lib-smtp: server: Fix reporting of XCLIENT capability in EHLO response. The trusted connection check logic was inverted.

lib-smtp: server: Fix overwriting a previously submitted reply. The submitted flag was not reset, nor was the replies_submitted counter decreased. This caused assertion failures.

lib-smtp: server: Use smtp_server_reply_submit_duplicate() in smtp_server_reply_allv(). This is more efficient, since it doesn't need to compose new content for each reply.

lib-smtp: server: Add function that allows submitting duplicate replies. The smtp_server_reply_submit_duplicate() function submits a new reply at the specified index with content duplicated from an existing earlier reply.

lib-smtp: server: Move allocation of command replies to a separate function.

lib-smtp: server: Put reply content in a separate struct. This allows duplicate replies to share content for a multi-reply command.

lib-smtp: server: Allow part of the replies to be already sent in smtp_server_reply_allv(). This is not something observed to happen in existing code, but it is best to deal with this properly, as it could happen in the future.

Updated copyright notices to include the year 2018.

lib-smtp: Implemented SMTP server.