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/syscall_darwin.go

@@ -14,6 +14,7 @@ package unix
 
 import (
 	errorspkg "errors"
+	"syscall"
 	"unsafe"
 )
 
@@ -208,7 +209,7 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 
 //sys	kill(pid int, signum int, posix int) (err error)
 
-func Kill(pid int, signum Signal) (err error) { return kill(pid, int(signum), 1) }
+func Kill(pid int, signum syscall.Signal) (err error) { return kill(pid, int(signum), 1) }
 
 /*
  * Exposed directly