Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

vendor/github.com/urfave/cli/v2/cli.go

@@ -0,0 +1,25 @@
+// Package cli provides a minimal framework for creating and organizing command line
+// Go applications. cli is designed to be easy to understand and write, the most simple
+// cli application can be written as follows:
+//
+//	func main() {
+//		(&cli.App{}).Run(os.Args)
+//	}
+//
+// Of course this application does not do much, so let's make this an actual application:
+//
+//	func main() {
+//		app := &cli.App{
+//	  		Name: "greet",
+//	  		Usage: "say a greeting",
+//	  		Action: func(c *cli.Context) error {
+//	  			fmt.Println("Greetings")
+//	  			return nil
+//	  		},
+//		}
+//
+//		app.Run(os.Args)
+//	}
+package cli
+
+//go:generate make -C cmd/urfave-cli-genflags run