unix: add GetsockoptTCPInfo

Change-Id: I190098a4a969a706c555e48d2ad938d41017601e
Reviewed-on: https://go-review.googlesource.com/39195
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -744,6 +744,13 @@ func GetsockoptUcred(fd, level, opt int) (*Ucred, error) {
 	return &value, err
 }
 
+func GetsockoptTCPInfo(fd, level, opt int) (*TCPInfo, error) {
+	var value TCPInfo
+	vallen := _Socklen(SizeofTCPInfo)
+	err := getsockopt(fd, level, opt, unsafe.Pointer(&value), &vallen)
+	return &value, err
+}
+
 func SetsockoptIPMreqn(fd, level, opt int, mreq *IPMreqn) (err error) {
 	return setsockopt(fd, level, opt, unsafe.Pointer(mreq), unsafe.Sizeof(*mreq))
 }