unix/linux: Prefixmsg and PrefixCacheinfo structs

These structs and enumeration constants are used
to parse RTM_NEWPREFIX netlink messages.

Fixes golang/go#77662

Change-Id: I1e7a92e771701c0c790c15e58ad78cd8fe1e5cf5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/746420
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>