os: emulate plan 9 libc in stat

os: emulate plan 9 libc in stat

This change is a recreation of the CL written
by Nick Owens on http://golang.org/cl/150730043.

If the stat buffer is too short, the kernel
informs us by putting the 2-byte size in the
buffer, so we read that and try again.

This follows the same algorithm as /sys/src/libc/9sys/dirfstat.c.

Fixes #8781.

Change-Id: I01b4ad3a5e705dd4cab6673c7a119f8bef9bbd7c
Reviewed-on: https://go-review.googlesource.com/3281
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>