Avoid duplicate kernel options

[JokerQyou]

This is a follow-up to #557. I would like to know if the proposed changes are acceptable.

The -ko volname=XXX does not work, because gocryptfs sets volname to directory base name of the mountpoint first, then expect to override it with duplicate option-value pair. Relevant codes:

gocryptfs/mount.go

Lines 437 to 442 in f06f27e

	// Add a volume name if running osxfuse. Otherwise the Finder will show it as
	// something like "osxfuse Volume 0 (gocryptfs)".
	if runtime.GOOS == "darwin" {
	volname := strings.Replace(path.Base(args.mountpoint), ",", "_", -1)
	mOpts.Options = append(mOpts.Options, "volname="+volname)
	}

gocryptfs/mount.go

Lines 467 to 473 in f06f27e

	// Add additional mount options (if any) after the stock ones, so the user has
	// a chance to override them.
	if args.ko != "" {
	parts := strings.Split(args.ko, ",")
	tlog.Debug.Printf("Adding -ko mount options: %v", parts)
	mOpts.Options = append(mOpts.Options, parts...)
	}

Basically, it would produce -o volname=BaseNameOfMountPoint,volname=CustomVolName option for macFUSE.
This appears logical at first glance, but it doesn't work as expected. macFUSE only reads the initial volname value set by gocryptfs and does not check for updates to this option later on.

I swapped these two code blocks and test on macOS Sonoma and it works as expected, the volname option can now be overridden correctly.

I think it's worth rewriting this part of gocryptfs so it only sets each option exactly once - use only user-provided option if possible, only inject stock options if they are not provided.