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_freebsd_amd64.go

@@ -4,7 +4,10 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
 
 func Getpagesize() int { return 4096 }
 
@@ -55,4 +58,4 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 	return
 }
 
-func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err Errno)
+func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)