net: add a method to net.TCPConn, to send MSG_OOB "urgent data"

[rin01]

Usually, when a user has sent a request to a server, and the request is running from a too long time, the user can send a "cancel" command by sending an out-of-band data to the server.

The server can then immediately take the appropriate measure.

Is it possible to add a method to net.TCPConn, that would call the C function "send(..., MSG_OOB)" ?

[ianlancetaylor]

CC @mikioh

[mikioh]

There is #7882, but no concrete plan yet.

[mikioh]

Merged into #7882.