Reduce pointer indirection for global list_t variables

Reduce pointer indirection for global list_t variables

Converted cmdline_conf, connection_list, outgoing_list from
pointer-to-structs to structs.

Created list_empty_list for these structs. This is necessary,
because list_delete_list frees the supplied list_t pointer.

Part of https://github.com/gsliepen/tinc/issues/294