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