global: don't cast NULL to void * NULL is guaranteed to be a void * thanks to dd6043c05e32a8e8db1233ed711a2c74d1477a89.
Updated copyright notices to include the year 2018.
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: Added max_recipients setting, which enforces a recipient limit per-transaction.
lib-smtp: Created test-smtp-server-errors, which tests the server's error handling.