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/netstat_exec.go

@@ -8,9 +8,8 @@ package portlist
 
 import (
 	"fmt"
+	"os/exec"
 	"strings"
-
-	exec "tailscale.com/tempfork/osexec"
 )
 
 var osHideWindow func(*exec.Cmd) // non-nil on Windows; see portlist_windows.go