netlink: use domain specific send buffer

[glebius]

Summary

Instead of using generic socket code, create Netlink specific socket
buffer. It is a simple TAILQ of writes that came from userland. This
saves us one memory allocation that could fail and one memory copy.

[melifaro]

sys/netlink/netlink_io.c

236–250

any concerns on copying the previous approach? (grab all Q and the insert back if we hit the limit)?