netlink: temporary skip snl(3) capped test.

netlink: fix capped uncapped ack handling in snl(3).

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D39144
MFC after: 2 weeks

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

netlink: add snl(3) default parsers for routes and links.

This change adds the "default" parsers of _all_ route/link attributes
exported by the kernel.
It removes the need to declare similar parsers in the userland applications,
simplifying their logic.

Differential Revision: https://reviews.freebsd.org/D38979
MFC after: 2 weeks

netlink: add snl(3) - simple netlink library

Reviewed by: bapt, pauamma
Differential Revision: https://reviews.freebsd.org/D37736