os: don't return Chmod's error from Mkdir and OpenFile

os: don't return Chmod's error from Mkdir and OpenFile

Mkdir and OpenFile call Chmod internally on *BSD and Solaris,
because these OSes don't handle the sticky bit correctly.

However Chmod's error should be ignored. It shouldn't hide
the fact that a file itself is created.

Fixes #8383

Change-Id: Ia2e0b2ba72712d73a0a48ba5a263432e0fff31a5
Reviewed-on: https://go-review.googlesource.com/2057
Reviewed-by: Russ Cox <rsc@golang.org>