os: guarantee min buffer size for ReadFile reads on /proc-like files

os: guarantee min buffer size for ReadFile reads on /proc-like files

For instance, this fixes os.ReadFile on plan9's /net/iproute file.

But it's not necessarily plan9-specific; Linux /proc and /sys filesystems
can exhibit the same problems.

Fixes #72080

Change-Id: I60b035913f583a91c6d84df95a6ea7b7ec2b3c92
Reviewed-on: https://go-review.googlesource.com/c/go/+/654315
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>