netlink: improve snl(3)

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