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/asm_netbsd_386.s

@@ -5,7 +5,7 @@
 // TODO(rsc): Rewrite all nn(SP) references into name+(nn-8)(FP)
 // so that go vet can check that they are correct.
 
-#include "../../cmd/ld/textflag.h"
+#include "../../cmd/ld/textflag.h" // TODO: how to refer to this?
 
 //
 // System call support for 386, NetBSD