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

unix/mksysnum_linux.pl

@@ -11,7 +11,7 @@ print <<EOF;
 // $command
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
-package syscall
+package unix
 
 const(
 EOF