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/README.md

@@ -0,0 +1,19 @@
+# cli
+
+[![Run Tests](https://github.com/urfave/cli/actions/workflows/cli.yml/badge.svg?branch=v2-maint)](https://github.com/urfave/cli/actions/workflows/cli.yml)
+[![Go Reference](https://pkg.go.dev/badge/github.com/urfave/cli/v2.svg)](https://pkg.go.dev/github.com/urfave/cli/v2)
+[![Go Report Card](https://goreportcard.com/badge/github.com/urfave/cli/v2)](https://goreportcard.com/report/github.com/urfave/cli/v2)
+[![codecov](https://codecov.io/gh/urfave/cli/branch/v2-maint/graph/badge.svg?token=t9YGWLh05g)](https://app.codecov.io/gh/urfave/cli/tree/v2-maint)
+
+cli is a simple, fast, and fun package for building command line apps in Go. The
+goal is to enable developers to write fast and distributable command line
+applications in an expressive way.
+
+## Documentation
+
+More documentation is available in [`./docs`](./docs) or the hosted
+documentation site at <https://cli.urfave.org>.
+
+## License
+
+See [`LICENSE`](./LICENSE)