credentials: Serve(): use "Name instead of "os.Args[0]" for usage output

GNU guidelines describes; https://www.gnu.org/prep/standards/html_node/_002d_002dversion.html#g_t_002d_002dversion

    The program’s name should be a constant string; don’t compute it from argv[0].
    The idea is to state the standard or canonical name for the program, not its
    file name.

Although the above recommendation is for `--version` output, it probably makes
sense to do the same for the "usage" output.

Before this change:

    /usr/local/bin/docker-credential-osxkeychain invalid command
    Usage: /usr/local/bin/docker-credential-osxkeychain <store|get|erase|list|version>

    /Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain invalid command
    Usage: /Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain <store|get|erase|list|version>

With this patch:

    /usr/local/bin/docker-credential-osxkeychain invalid command
    Usage: docker-credential-osxkeychain <store|get|erase|list|version>

    /Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain invalid command
    Usage: docker-credential-osxkeychain <store|get|erase|list|version>

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

credentials/credentials.go

@@ -51,7 +51,7 @@ func SetCredsLabel(label string) {
 func Serve(helper Helper) {
 	var err error
 	if len(os.Args) != 2 {
-		err = fmt.Errorf("Usage: %s <store|get|erase|list|version>", os.Args[0])
+		err = fmt.Errorf("Usage: %s <store|get|erase|list|version>", Name)
 	}
 
 	if err == nil {