tempfork/osexec: remove old fork of os/exec

This package was a temporary fork of os/exec to fix an EINTR loop
bug that was fixed upstream for Go 1.15 in
8c1db77a92
(https://go-review.googlesource.com/c/go/+/232862), in
src/os/exec_unix.go:

8c1db77a92 (diff-72072cbd53a7240debad8aa506ff7ec795f9cfac7322e779f9bac29a4d0d0bd4)

portlist/portlist_macos.go

@@ -12,11 +12,10 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"strings"
 	"sync/atomic"
 	"time"
-
-	exec "tailscale.com/tempfork/osexec"
 )
 
 // We have to run netstat, which is a bit expensive, so don't do it too often.