env: update Plan 9 image

This new Plan 9 image provides various fixes to the Plan 9 kernel.

The most notable change is a fix to the pread system call, so
pread will not update the channel offset when reading a file.

Updates golang/go#11194.
Updates golang/go#14534.

Change-Id: Ia8f537b3559fbc98a191bda836cd3e0035e16eec
Reviewed-on: https://go-review.googlesource.com/22319
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

env/plan9-386/make.bash

@@ -6,7 +6,7 @@ set -e
 
 # Download Plan 9
 if ! sha1sum -c plan9-gce.iso.sha1; then
-  curl --fail -O http://9legacy.org/download/go/2015-02-26/plan9-gce.iso.bz2
+  curl --fail -O http://9legacy.org/download/go/2016-05-01/plan9-gce.iso.bz2
   bunzip2 plan9-gce.iso.bz2
   sha1sum -c plan9-gce.iso.sha1
 fi
@@ -135,6 +135,12 @@ send "\n"
 expect -exact "term% "
 send "9fat:\n"
 
+expect -exact "term% "
+send "sed /^debugboot'='/d /n/9fat/plan9.ini >/tmp/plan9.ini\n"
+
+expect -exact "term% "
+send "mv /tmp/plan9.ini /n/9fat/plan9.ini\n"
+
 expect -exact "term% "
 send "sed s/9pcf/9pccpuf/ /n/9fat/plan9.ini >/tmp/plan9.ini\n"
 
@@ -235,6 +241,8 @@ expect -exact "done halting"
 exit
 EOF
 
+echo
+
 # Create Compute Engine disk image.
 echo "Archiving disk.raw... (this may take a while)"
 tar -Szcf plan9-386-gce.tar.gz disk.raw

env/plan9-386/plan9-gce.iso.sha1

@@ -1 +1 @@
-d277abfb4431ae1ca2cbf93d3c0b2cacdad6f166  plan9-gce.iso
+89d98820b839cd37ab00af8c2252665d94913235  plan9-gce.iso