mount-util: tweak flags decoding in mount_verbose_full()

mount-util: tweak flags decoding in mount_verbose_full()

Fine-tune the decoding of mount options in mount_verbose_full() to
provide more helpful log output:

1. decode changing of propagation changes
2. discern changing of superblock flags/mount option string from mount
   flags
3. don't check secondary fields when deciding which mount op is
   executed, only the flags decide that.