syscall: add GetsockoptTCPInfo()

[gopherbot]

by willchan@chromium.org:

syscall.TCPInfo exists but there's no accessor for it.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.3maybe, os-linux.

Status changed to Accepted.

[mikioh]

Comment 2:

I hope that the API would wrap raw tcp_info struct and provide some abstracted views of
TCP across over platforms, for now both Linux and FreeBSD, because FreeBSD guys are
saying that http://svnweb.freebsd.org/base?view=revision&revision=212765 (yup, they have
another CC module API).

[ianlancetaylor]

Comment 3:

My first thought is that that kind of abstracted view is out of scope for the syscall
package.  We don't do it for anything else.

[mikioh]

Comment 4:

roger that.

[rsc]

Comment 5:

Status changed to WorkingAsIntended.