Use the socket option constants defined in the golang.org/x/sys/unix
package instead of duplicating them in this package.
Note that for now, sysIPV6_TCLASS, sysIPV6_PATHMTU, sysIPV6_PKTINFO,
sysIPV6_HOPLIMIT and sysIPV6_NEXTHOP still need to be defined because
they are used in code shared with other platforms.
Change-Id: I40896c6c191aade35e3f9f1fb1a624f772e7479a
Reviewed-on: https://go-review.googlesource.com/c/net/+/306071
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>