exec_plan9.go:
. Adjusted return argument to match other changes.
#mksyscall.pl:
. Replaced "err = e1" with "err = NewError(e1)".
* Change abandoned, Russ made a better suggestion involving
  syscall_plan9.go.
syscall_plan9.go:
. Removed redundant "err = nil" lines.
. Adjusted //sys lines for mksyscall.pl.
* Replaced "err string" with "err ErrorString" in return arguments.
zsyscall_plan9_386.go:
. This module ought to be generated, but as it exists in the
  repository, I rebuilt it and checked that it matched expectations.
  Anybody is welcome to remove this from the repository if
  they feel it should go, but remember that not all Plan 9
  installations have a working Perl.

R=rsc
CC=ality, golang-dev
https://golang.org/cl/5411046