Validate additional listen opts

Validate additional listen opts

The options "inet", "inet6" and "backlog" are valid listen options, but are
currently logged as errors (even though they do work):

2018-02-28 16:08:44.141 [error] <0.338.0>@ejabberd_listener:validate_module_option:630 unknown listen option 'backlog' for 'ejabberd_c2s' will be likely ignored, available options are: access, shaper, certfile, ciphers, dhfile, cafile, client_cafile, protocol_options, tls, tls_compression, starttls, starttls_required, tls_verify, zlib, max_fsm_queue

This adds the necessary validators so they are correctly recognized.