os: delete os.EINVAL and so on

The set of errors forwarded by the os package varied with system and
was therefore non-portable.
Three helpers added for portable error checking: IsExist, IsNotExist, and IsPermission.
One or two more may need to come, but let's keep the set very small to discourage
thinking about errors that way.

R=mikioh.mikioh, gustavo, r, rsc
CC=golang-dev
https://golang.org/cl/5672047

src/pkg/os/file_unix.go

@@ -90,7 +90,7 @@ func (f *File) Close() error {
 
 func (file *file) close() error {
 	if file == nil || file.fd < 0 {
-		return EINVAL
+		return syscall.EINVAL
 	}
 	var err error
 	if e := syscall.Close(file.fd); e != nil {