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

Updated copyright notices to include the year 2018.

lib-smtp: client: Add assertion to smtp_client_command_write(). Makes sure it is not used after the command is submitted.

lib-smtp: client: Fix smtp_client_command_name_equals() to work properly after the command is submitted. At command submission, CRLF is appended to command data. This messed up the name comparison. This in turn caused a spurious QUIT command to be sent to the server at connection close.

lib-smtp: client: Make smtp_client_connection_commands_abort() more reliable by copying the command lists. Copy the current lists of queued and waiting commands and reference each command before calling smtp_client_command_abort(). Aborting one command can cause other dependent commands to be aborted (in a transaction or from submission service), which could have caused trouble in this function.

lib-smtp: client: Make smtp_client_connection_commands_fail_reply() more reliable by copying the command lists. Copy the current lists of queued and waiting commands and reference each command before calling smtp_client_command_fail_reply(). Failing one command can cause other dependent commands to be aborted (in a transaction or from submission service), which could have caused trouble in this function. Problems would likely occur at connection disconnect.

lib-smtp: fix uoff_t vs. size_t confusion

lib-smtp: client: Fixed sending payload with unknown size using the CHUNKING capability.

lib-smtp: client: command: Implemented callback that is called when command is fully sent.

lib-smtp: Implemented SMTP client.