Opening /dev/stdin can sometimes fail. For example, in the acme editor,
executing "Edit ,|gofmt" fails with:

	open /dev/stdin: no such device or address

Executing "Edit ,|ls -l /dev/stdin /proc/self/fd/0" gives:

	lrwxrwxrwx 1 root root  15 2009-09-07 02:17 /dev/stdin -> /proc/self/fd/0
	lrwx------ 1 fhs  users 64 2009-11-26 22:05 /proc/self/fd/0 -> socket:[5528230]

(This is my first change, and I've signed the individual contributor license agreement.)

R=rsc, gri
CC=golang-dev
https://golang.org/cl/162041