Add a Getsockopt variant returning a string. This can e.g. be used to
retrieve a peer's SELinux or AppArmor labels using SO_PEERSEC.

Fixes golang/go#16374

Change-Id: Ia266bad1b0aef94ce19d378719ce1c1c976b4cc5
Reviewed-on: https://go-review.googlesource.com/83275
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>