Use unix.Getstatfs and its associated Statfs_t type instead of
implementing them locally in this package. This allows to use 64-bit
inode fields on FreeBSD 12 while still keeping backwards compatibility
for old FreeBSD versions, as unix.Getfsstat will use the correct syscall
number and data structure version and convert its result
correspondingly.
Also see https://golang.org/cl/136816 for details.
