There was a breaking change in upstream golang.org/x/sys/unix which
changed the type of some Statvfs members, see #853. As it looks like
this change won't be reverted, adjust gopsutil to work with it and allow
to build against the latest version of x/sys/unix.
All functions used from github.com/shirou/w32 are also available from
golang.org/x/sys/windows which is already used in other places. Convert
the remaining usages to use the functions from x/sys/windows.
