PR go/69966
syscall: Add new Getsockopt functions.
Add GetsockoptICMPv6Filter, GetsockoptIPv6MTUInfo, GetsockoptUcred as
appropriate. These functions exist in the master library.
For GCC PR 69966.
Reviewed-on: https://go-review.googlesource.com/19960
From-SVN: r233747