Old behavior: 10 consecutive EPIPE errors on any descriptor cause the
program to exit with a SIGPIPE signal.

New behavior: an EPIPE error on file descriptors 1 or 2 cause the
program to raise a SIGPIPE signal.  If os/signal.Notify was not used to
catch SIGPIPE signals, this will cause the program to exit with SIGPIPE.
An EPIPE error on a file descriptor other than 1 or 2 will simply be
returned from Write.

Fixes #11845.
Update #9896.

Change-Id: Ic85d77e386a8bb0255dc4be1e4b3f55875d10f18
Reviewed-on: https://go-review.googlesource.com/18151Reviewed-by: Russ Cox <rsc@golang.org>