This is to provide an explicit option whether to use the system proxy setting for SMTP connections, as described in T7430.
If the system proxy settings are set to use the environment variables http_proxy etc (for compatibility with non-KDE applications), then they are also used for SMTP connections. This is rarely required and will fail unless the proxy is transparent enough. See https://bugs.kde.org/show_bug.cgi?id=379539 and related bugs.
This change adds a SMTP option to use the proxy for connection (the default is off). This is the core part of the change in KSmtp; the configuration option and the visible GUI part in KMailTransport will follow in another diff.