credentials: Serve(): implement "--version, -v", and "--help, -h" flags

As recommended in the GNU documentation;

- https://www.gnu.org/prep/standards/standards.html#g_t_002d_002dversion
- https://www.gnu.org/prep/standards/standards.html#g_t_002d_002dhelp

With this patch:

    $ docker-credential-osxkeychain --version
    docker-credential-osxkeychain (github.com/docker/docker-credential-helpers) v0.7.0-51-g26c426e.m

    $ docker-credential-osxkeychain -v
    docker-credential-osxkeychain (github.com/docker/docker-credential-helpers) v0.7.0-51-g26c426e.m

    $ docker-credential-osxkeychain --help
    Usage: docker-credential-osxkeychain <store|get|erase|list|version>

    $ docker-credential-osxkeychain -h
    Usage: docker-credential-osxkeychain <store|get|erase|list|version>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

credentials/credentials.go

@@ -54,6 +54,15 @@ func Serve(helper Helper) {
 		os.Exit(1)
 	}
 
+	switch os.Args[1] {
+	case "--version", "-v":
+		_ = PrintVersion(os.Stdout)
+		os.Exit(0)
+	case "--help", "-h":
+		_, _ = fmt.Fprintln(os.Stdout, usage())
+		os.Exit(0)
+	}
+
 	if err := HandleCommand(helper, os.Args[1], os.Stdin, os.Stdout); err != nil {
 		_, _ = fmt.Fprintln(os.Stdout, err)
 		os.Exit(1)