submission: Add settings that configure the connect and command reply timeouts.

submission: Enable verbose reporting of relay server certificate problems.

Updated copyright notices to include the year 2018.

submission: Block server input handling until proxy connection to relay server completes. This makes sure we are aware of the full extent of the backend server's capabilities, before we start handling commands. Before, only EHLO commands were blocked this way, but this did not account for MAIL/RCPT parameters, which are equally dependent. Fixes problems with clients that send no EHLO after AUTH, like Thunderbird.

lib-smtp: server: Remove implicit smtp_server_connection_start() from smtp_server_connection_login().

lib-smtp: server: Moved ssl_start argument from smtp_server_connection_start() to smtp_server_connection_create(). This frees smtp_server_connection_start() for general use.

submission: Restructure handling of QUIT command. - Avoid explicitly proxying QUIT command when the proxy connection is not ready: in that case the SMTP client connection will just send QUIT if appropriate, without waiting for reply. - Add timeout for proxied QUIT command, so that there are no problems when the relay server hangs after QUIT (addresses FIXME).

submission: Initialize proxy connection before starting the SMTP server connection. The smtp_server_connection_login() function gets pre-login connection data from login service, which can contain commands. The execution of commands expects the proxy connection object to be initialized.

submission: remove unnecessary casts Casting from a void * to any pointer type is automatic and doesn't require an explicit cast.

Implemented SMTP submission proxy service.