Extract device major/minor number on all the BSDs and set Devmajor and
Devminor in FileInfoHeader. Code based on the corresponding Major/Minor
implementations in golang.org/x/sys/unix.

Change-Id: Ieffa7ce0cdbe6481950de666b2f5f88407a32382
Reviewed-on: https://go-review.googlesource.com/63470Reviewed-by: Joe Tsai <joetsai@google.com>