odd handling of `image_mountdir`

I've been spending a lot of time in lcfs_mount lately and there are a few weird issues around options->image_mountdir.

• this is specified by the user as a const char * but we internally cast it to char * so that we can pass it to some functions which require it to be char *. But those functions don't modify it and can be changed to consume const char * without issue.

• in the normal exit path (error or not error) we check that we were the ones that created the temporary directory before we remove it:

	if (created_tmpdir) {
		rmdir(imagemount);
        }

• but we have an early exit path which removes it unconditionally (if we failed to mount the erofs)

	if (err < 0) {
		rmdir(imagemount);
		return err;
	}

• but maybe we just want to eliminate this feature entirely. It's used by nothing else inside of composefs itself. ostree uses it, though, so maybe we decide to keep it....

• but in that case, we should definitely improve the const-correctness issues and also fix the bug where we rmdir() the user's directory in some error cases but not others

• all of this goes a little bit towards how complicated the error handling in C in libcomposefs has become....

Start in lcfs_mount and trace every use of options->image_mountdir, including the early EROFS mount-failure path and the normal cleanup path. Check the ostree usage before deciding whether to retain the option; done means the chosen behavior is consistent, user-owned directories are not removed, and the const-correctness issues are addressed.