go.sys: update package names
Semi-automatic migration from package syscall to package {plan9,windows,unix}.
No builds attempted yet, but this gets a lot of noise behind us so subsequent
CLs will be more concise and easier to follow.
Subsequent CLs will have semantic content.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/121520043
diff --git a/windows/syscall.go b/windows/syscall.go
index f7473fd..66b12ea 100644
--- a/windows/syscall.go
+++ b/windows/syscall.go
@@ -2,22 +2,22 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package syscall contains an interface to the low-level operating system
-// primitives.  The details vary depending on the underlying system, and
-// by default, godoc will display the syscall documentation for the current
+// Package windows contains an interface to the low-level operating system
+// primitives.  OS details vary depending on the underlying system, and
+// by default, godoc will display the OS-specific documentation for the current
 // system.  If you want godoc to display syscall documentation for another
 // system, set $GOOS and $GOARCH to the desired system.  For example, if
 // you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
 // to freebsd and $GOARCH to arm.
-// The primary use of syscall is inside other packages that provide a more
+// The primary use of this package is inside other packages that provide a more
 // portable interface to the system, such as "os", "time" and "net".  Use
 // those packages rather than this one if you can.
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
 // These calls return err == nil to indicate success; otherwise
 // err is an operating system error describing the failure.
-// On most systems, that error has type syscall.Errno.
-package syscall
+// That error has type windows.Errno.
+package windows
 
 // StringByteSlice is deprecated. Use ByteSliceFromString instead.
 // If s contains a NUL byte this function panics instead of
@@ -25,7 +25,7 @@
 func StringByteSlice(s string) []byte {
 	a, err := ByteSliceFromString(s)
 	if err != nil {
-		panic("syscall: string with NUL passed to StringByteSlice")
+		panic("windows: string with NUL passed to StringByteSlice")
 	}
 	return a
 }