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

windows/mksyscall.pl

@@ -4,7 +4,7 @@
 # license that can be found in the LICENSE file.
 
 # This program reads a file containing function prototypes
-# (like syscall_darwin.go) and generates system call bodies.
+# (like syscall_windows.go) and generates system call bodies.
 # The prototypes are marked by lines beginning with "//sys"
 # and read like func declarations if //sys is replaced by func, but:
 #	* The parameter lists must give a name for each argument.
@@ -304,7 +304,7 @@ print <<EOF;
 // $cmdline
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
-package syscall
+package windows
 
 import "unsafe"