go.sys/unix: use syscall.Errno for errors

If we use a local type, it won't compare properly with errors from
the rest of the standard library. Errors are the one type from syscall
that propagates through the system, so it's important to have only
one type for them.

Ditto for syscall.Signal.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/123490043

unix/zsyscall_openbsd_amd64.go

@@ -3,7 +3,10 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
@@ -652,7 +655,7 @@ func Issetugid() (tainted bool) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Kill(pid int, signum Signal) (err error) {
+func Kill(pid int, signum syscall.Signal) (err error) {
 	_, _, e1 := Syscall(SYS_KILL, uintptr(pid), uintptr(signum), 0)
 	if e1 != 0 {
 		err = e1