os: show how to print permission bits in octal

Permission bits are most commonly viewed in string form (rwx-- etc) or
in octal form (0755), but the latter is relatively rare in Go.
Demonstrate how to print a FileMode in readable octal format.

Change-Id: I41feb801bcecb5077d4eabafdea27c149fc179a1
Reviewed-on: https://go-review.googlesource.com/c/154423


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>