os: add Sys method to FileInfo

[rsc]

add Sys() interface{} to FileInfo.

This will make it a little easier on the people who want to get
at the underlying syscall.Stat_t, but it also makes it possible at all
to get at the underlying info for wrappers like zip that must use
another type because the basic type has a Name string field
that would conflict with a Name() string method.

[rsc]

Comment 1:

Gustavo did this.

Status changed to Fixed.