netlink: cope with growing requests

If a request ends up growing beyong the initially allocated space the
netlink functions (such as snl_add_msg_attr_u32()) will allocate a
new buffer. This invalidates the header pointer we can have received
from snl_create_msg_request(). Always use the hdr returned by
snl_finalize_msg().

Reviewed by: melifaro
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42223

netlink: add snl(3) support for listing genetlink multicast groups

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D40282
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