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_windows.go

@@ -5,11 +5,11 @@
 package portlist
 
 import (
+	"os/exec"
 	"syscall"
 	"time"
 
 	"golang.org/x/sys/windows"
-	exec "tailscale.com/tempfork/osexec"
 )
 
 // Forking on Windows is insanely expensive, so don't do it too often.