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_linux_arm.go

@@ -4,7 +4,10 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
 
 func Getpagesize() int { return 4096 }
 
@@ -25,7 +28,7 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 
 // Underlying system call writes to newoffset via pointer.
 // Implemented in assembly to avoid allocation.
-func seek(fd int, offset int64, whence int) (newoffset int64, err Errno)
+func seek(fd int, offset int64, whence int) (newoffset int64, err syscall.Errno)
 
 func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 	newoffset, errno := seek(fd, offset, whence)