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: transaction: Add function that finds duplicate recipients. Duplicate means that both the path and parameters are equal.

lib-smtp: server: Add APIs for halting and resuming connection command handling. Also adds function to start the connection in a pending (halted) state. This way the greeting can already be sent (over SSL if needed), while deferring command handling until some external activity is completed.

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

lib-smtp: server: Moved smtp_server_connection_start() after smtp_server_connection_login(). Also fixes an indent problem in the header comment implicitly.

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.

lib-smtp: server: Add smtp_server_connection_set_ssl_streams(). It wraps smtp_server_connection_set_streams() and additionally marks the connection as SSL-secured.

lib-smtp: server: Change conn_start_tls() connection callback to return a success status.

lib-smtp: server: Added max_recipients setting, which enforces a recipient limit per-transaction.

lib-smtp: server: command: Added smtp_server_command_input_unlock(). Allows unlock input for a command that was earlier locked with smtp_server_command_input_lock().

lib-smtp: Implemented SMTP server.