improve default header

 * use `csaf_distribution` with an underscore as underscores
   are allowed by RFC9110 and it is more consistent as it is used
   with underscore at other places.
 * change example to `VERSION` to indicate that this is dynamic.

docs/csaf_checker.md

@@ -30,7 +30,7 @@ Help Options:
 
 Will check all given _domains_, by trying each as a CSAF provider.
 
-If no user agent is specified with `--header=user-agent:custom-agent/1.0` then the default agent in the form of `csaf-distribution/version` is sent.
+If no user agent is specified with `--header=user-agent:custom-agent/1.0` then the default agent in the form of `csaf_distribution/VERSION` is sent.
 
 If a _domain_ starts with `https://` it is instead considered a direct URL to the `provider-metadata.json` and checking proceeds from there.
 

docs/csaf_downloader.md

@@ -40,7 +40,7 @@ Help Options:
 
 Will download all CSAF documents for the given _domains_, by trying each as a CSAF provider.
 
-If no user agent is specified with `--header=user-agent:custom-agent/1.0` then the default agent in the form of `csaf-distribution/version` is sent.
+If no user agent is specified with `--header=user-agent:custom-agent/1.0` then the default agent in the form of `csaf_distribution/VERSION` is sent.
 
 If a _domain_ starts with `https://` it is instead considered a direct URL to the `provider-metadata.json` and downloading procedes from there.
 

util/client.go

@@ -64,7 +64,7 @@ func (hc *HeaderClient) Do(req *http.Request) (*http.Response, error) {
 
 	// Use default user agent if none is set
 	if userAgent := hc.Header.Get("User-Agent"); userAgent == "" {
-		req.Header.Add("User-Agent", "csaf-distribution/"+SemVersion)
+		req.Header.Add("User-Agent", "csaf_distribution/"+SemVersion)
 	}
 	return hc.Client.Do(req)
 }