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/unix/zsysnum_darwin_amd64.go b/unix/zsysnum_darwin_amd64.go
index abdef77..645426a 100644
--- a/unix/zsysnum_darwin_amd64.go
+++ b/unix/zsysnum_darwin_amd64.go
@@ -1,7 +1,7 @@
-// mksysnum_darwin.pl /usr/include/sys/syscall.h
+// mksysnum_darwin.pl /usr/include/sys/unix.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
-package syscall
+package unix
 
 const (
 	SYS_SYSCALL                        = 0