cli, lib, lang: Port to new cli library

The new version of the urfave/cli library is moving to generics, and
it's completely unclear to me why this is an improvement. Their new API
is very complicated to understand, which for me, defeats the purpose of
golang.

In parallel, I needed to do some upcoming cli API refactoring, so this
was a good time to look into new libraries. After a review of the
landscape, I found the alexflint/go-arg library which has a delightfully
elegant API. It does have a few rough edges, but it's otherwise very
usable, and I think it would be straightforward to add features and fix
issues.

Thanks Alex!

cli/util/util.go

@@ -20,9 +20,29 @@ package util
 
 import (
 	"strings"
+
+	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
+// Error is a constant error type that implements error.
+type Error string
+
+// Error fulfills the error interface of this type.
+func (e Error) Error() string { return string(e) }
+
+const (
+	// MissingEquals means we probably hit the parsing bug.
+	// XXX: see: https://github.com/alexflint/go-arg/issues/239
+	MissingEquals = Error("missing equals sign for list element")
+)
+
+// CliParseError returns a consistent error if we have a CLI parsing issue.
+func CliParseError(err error) error {
+	return errwrap.Wrapf(err, "cli parse error")
+}
+
 // Flags are some constant flags which are used throughout the program.
+// TODO: Unify this with Debug and Logf ?
 type Flags struct {
 	Debug   bool // add additional log messages
 	Verbose bool // add extra log message output
@@ -33,6 +53,7 @@ type Data struct {
 	Program string
 	Version string
 	Copying string
+	Tagline string
 	Flags   Flags
 	Args    []string // os.Args usually
 }