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/mksyscall_solaris.pl

@@ -264,7 +264,7 @@ package $package
 import "unsafe"
 EOF
 
-print "import \"syscall\"\n" if $package ne "syscall";
+print "import \"unix\"\n" if $package ne "unix";
 
 print <<EOF;