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/exec_unix.go

@@ -29,7 +29,7 @@ const (
 // (WNOHANG etc.) affect the behavior of the Wait call.
 func (p *Process) Wait(options int) (w *Waitmsg, err error) {
 	if p.Pid == -1 {
-		return nil, EINVAL
+		return nil, syscall.EINVAL
 	}
 	var status syscall.WaitStatus
 	var rusage *syscall.Rusage