Summary:
* add snl_send_message() as a convenient send wrapper
* add signed integer parsers
* add snl_read_reply_code() to simplify operation result checks
* add snl_read_reply_multi() to simplify reading multipart messages
* add snl_create_genl_msg_request()
* add snl_get_genl_family() to simplify family name->id resolution
* add tests for some of the functionality
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D39092
MFC after: 2 weeks
