[release-branch.go1.8] net, net/http: adjust time-in-past constant even earlier

The aLongTimeAgo time value in net and net/http is used to cancel
in-flight read and writes. It was set to time.Unix(233431200, 0)
which seemed like far enough in the past.

But Raspberry Pis, lacking a real time clock, had to spoil the fun and
boot in 1970 at the Unix epoch time, breaking assumptions in net and
net/http.

So change aLongTimeAgo to time.Unix(1, 0), which seems like the
earliest safe value. I don't trust subsecond values on all operating
systems, and I don't trust the Unix zero time. The Raspberry Pis do
advance their clock at least. And the reported problem was that Hijack
on a ResponseWriter hung forever, waiting for the connection read
operation to finish. So now, even if kernel + userspace boots in under
a second (unlikely), the Hijack will just have to wait for up to a
second.

Updates #19747
Fixes #19771 (backport to Go 1.8.x)

Change-Id: Id59430de2e7b5b5117d4903a788863e9d344e53a
Reviewed-on: https://go-review.googlesource.com/38785
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit e83fc2e44336423dab94bfe74fad4c4e6a4703b3)
Reviewed-on: https://go-review.googlesource.com/38786Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/http.go

@@ -20,7 +20,7 @@ const maxInt64 = 1<<63 - 1
 
 // aLongTimeAgo is a non-zero time, far in the past, used for
 // immediate cancelation of network operations.
-var aLongTimeAgo = time.Unix(233431200, 0)
+var aLongTimeAgo = time.Unix(1, 0)
 
 // TODO(bradfitz): move common stuff here. The other files have accumulated
 // generic http stuff in random places.

src/net/net.go

@@ -468,7 +468,7 @@ func (e *OpError) Error() string {
 var (
 	// aLongTimeAgo is a non-zero time, far in the past, used for
 	// immediate cancelation of dials.
-	aLongTimeAgo = time.Unix(233431200, 0)
+	aLongTimeAgo = time.Unix(1, 0)
 
 	// nonDeadline and noCancel are just zero values for
 	// readability with functions taking too many parameters.