Rename catalog-zone-gen to catz across codebase

Update binary name, usage strings, generated file headers, tests,
and README. The generated BIND config header now includes the
install path: go install go.askask.com/catz@latest

bindconf.go

@@ -19,7 +19,7 @@ func generateBindConf(entries []ZoneEntry, cfg *Config) string {
 	})
 
 	var b strings.Builder
-	b.WriteString("# THIS FILE IS GENERATED BY catalog-zone-gen\n")
+	b.WriteString("# THIS FILE IS GENERATED BY catz (go install go.askask.com/catz@latest)\n")
 	b.WriteString("#=============================================\n")
 	b.WriteString("#\n")