Add kevent EVFILT_EMPTY for notification when a client has received all data

[kbowling]

Summary

This is used in production at Limelight Networks to monitor when a client has received all data in a socket buffer

Test Plan

A test daemon with usage instructions is available at https://github.com/kev009/filtempty. You will need to rewind one commit because the last commit uses TCP_INFO additions we have not yet upstreamed.

[hiren]

Can't actually review as an author of the patch. It'd be nice if someone else can look at it.

[gnn]

Do it.

[kbowling]

Updated to use sb_ccc. Add diff context

[bz]

otherwise looks good.

sys/kern/uipc_socket.c

3364 ↗

(On Diff #23949)

I'd suggest using sbused(&so->so_snd) there rather than directly accessing sb_ccc.

[kbowling]

Use sbused accessor per BZ