syscall: fix for Plan 9 build

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
diff --git a/src/pkg/syscall/syscall_plan9.go b/src/pkg/syscall/syscall_plan9.go
index c51c39a..292847d 100644
--- a/src/pkg/syscall/syscall_plan9.go
+++ b/src/pkg/syscall/syscall_plan9.go
@@ -38,8 +38,8 @@
 // creation of IPv6 sockets to return EAFNOSUPPORT.
 var SocketDisableIPv6 bool
 
-func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err string)
-func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err string)
+func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err ErrorString)
+func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err ErrorString)
 func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
 func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 
@@ -170,7 +170,6 @@
 func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 	newoffset, e := seek(0, fd, offset, whence)
 
-	err = nil
 	if newoffset == -1 {
 		err = NewError(e)
 	}
@@ -246,7 +245,7 @@
 	oldp := uintptr(unsafe.Pointer(StringBytePtr(old)))
 
 	var r0 uintptr
-	var e string
+	var e ErrorString
 
 	// bind(2) man page: If name is zero, everything bound or mounted upon old is unbound or unmounted.
 	if name == "" {
@@ -255,9 +254,8 @@
 		r0, _, e = Syscall(SYS_UNMOUNT, uintptr(unsafe.Pointer(StringBytePtr(name))), oldp, 0)
 	}
 
-	err = nil
 	if int(r0) == -1 {
-		err = NewError(e)
+		err = e
 	}
 	return
 }
@@ -288,7 +286,6 @@
 	if len(b) != 8 {
 		return -1, NewError("bad /dev/bintime format")
 	}
-	err = nil
 	nsec = int64(b[0])<<56 |
 		int64(b[1])<<48 |
 		int64(b[2])<<40 |
@@ -335,17 +332,17 @@
 	return make([]int, 0), nil
 }
 
-//sys	Dup(oldfd int, newfd int) (fd int, err Error)
-//sys	Open(path string, mode int) (fd int, err Error)
-//sys	Create(path string, mode int, perm uint32) (fd int, err Error)
-//sys	Remove(path string) (err Error)
-//sys	Pread(fd int, p []byte, offset int64) (n int, err Error)
-//sys	Pwrite(fd int, p []byte, offset int64) (n int, err Error)
-//sys	Close(fd int) (err Error)
-//sys	Chdir(path string) (err Error)
-//sys	Bind(name string, old string, flag int) (err Error)
-//sys	Mount(fd int, afd int, old string, flag int, aname string) (err Error)
-//sys	Stat(path string, edir []byte) (n int, err Error)
-//sys	Fstat(fd int, edir []byte) (n int, err Error)
-//sys	Wstat(path string, edir []byte) (err Error)
-//sys	Fwstat(fd int, edir []byte) (err Error)
+//sys	Dup(oldfd int, newfd int) (fd int, err error)
+//sys	Open(path string, mode int) (fd int, err error)
+//sys	Create(path string, mode int, perm uint32) (fd int, err error)
+//sys	Remove(path string) (err error)
+//sys	Pread(fd int, p []byte, offset int64) (n int, err error)
+//sys	Pwrite(fd int, p []byte, offset int64) (n int, err error)
+//sys	Close(fd int) (err error)
+//sys	Chdir(path string) (err error)
+//sys	Bind(name string, old string, flag int) (err error)
+//sys	Mount(fd int, afd int, old string, flag int, aname string) (err error)
+//sys	Stat(path string, edir []byte) (n int, err error)
+//sys	Fstat(fd int, edir []byte) (n int, err error)
+//sys	Wstat(path string, edir []byte) (err error)
+//sys	Fwstat(fd int, edir []byte) (err error)