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/exec_solaris.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package syscall
+package unix
 
 import (
 	"unsafe"
@@ -44,7 +44,7 @@ func write1(fd uintptr, buf uintptr, nbyte uintptr) (n uintptr, err Errno)
 // no rescheduling, no malloc calls, and no new stack segments.
 //
 // We call hand-crafted syscalls, implemented in
-// ../runtime/syscall_solaris.goc, rather than generated libc wrappers
+// runtime/syscall_solaris.goc, rather than generated libc wrappers
 // because we need to avoid lazy-loading the functions (might malloc,
 // split the stack, or acquire mutexes). We can't call RawSyscall
 // because it's not safe even for BSD-subsystem calls.