A user discovered that restic does not restore setuid/setgid/sticky file
attributes. This commit fixes that. The mode is stored in the Go format
as an uint32: https://golang.org/pkg/os/#FileMode
The syscall.Stat_t doesn't exist on Windows, so it is replaced by an interface,
which Windows can fill out, and field access is replaced by function calls.
Common Unix functionality is put into "node_unix.go", so there is less boilerplate.
Symlinks are skipped on Windows, since they require admin privileges.
