os: show how to print permission bits in octal
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,CAFxX/go,golang/go,josharian/go,josharian/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,josharian/go,josharian/go,cloudflare/go,golang/go,akutz/go,josharian/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,golang/go,josharian/go,akutz/go,cloudflare/go,akutz/go,josharian/go,golang/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,akutz/go