Ensure plugins can use PersistentPreRunE again.

I got a bit carried away in d4ced2ef77 ("allow plugins to have argument
which match a top-level flag.") and broke the ability of a plugin to use the
`PersistentPreRun(E)` hook on its top-level command (by unconditionally
overwriting it) and also broke the plugin framework if a plugin's subcommand
used those hooks (because they would shadow the root one). This could result in
either `dockerCli.Client()` returning `nil` or whatever initialisation the
plugin hoped to do not occuring.

This change revert the relevant bits and reinstates the requirement that a
plugin calls `plugin.PersistentPreRunE` if it uses that hook itself.

It is at least a bit nicer now since we avoid the need for the global struct
since the interesting state is now encapsulated in `tcmd` (and the closure).

In principal this could be done even more simply (by calling `tcmd.Initialize`
statically between `tcmd.HandleGlobalFlags` and `cmd.Execute`) however this has
the downside of _always_ initialising the cli (and therefore dialing the
daemon) even for the `docker-cli-plugin-metadata` command but also for the
`help foo` and `foo --help` commands (Cobra short-circuits the hooks in this
case).

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/examples/helloworld/main.go

@@ -51,7 +51,10 @@ func main() {
 		cmd := &cobra.Command{
 			Use:   "helloworld",
 			Short: "A basic Hello World plugin for tests",
-			PersistentPreRunE: func(_ *cobra.Command, _ []string) error {
+			PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
+				if err := plugin.PersistentPreRunE(cmd, args); err != nil {
+					return err
+				}
 				if preRun {
 					fmt.Fprintf(dockerCli.Err(), "Plugin PersistentPreRunE called")
 				}