libmount: exec mount helpers with posixly correct argument order

libmount: exec mount helpers with posixly correct argument order

This improves compatibility with non-gnu userspaces.

On systems where the libc provides posix getopt instead of gnu getopt,
mount helpers which use getopt to parse arguments will not parse
options which appear after non-option arguments. This patch ensures
mount/unmount work as expected in this situation.

mount: fix expected argument order for mount helpers in tests