mnet: restore support - XML parser knows about MNET_EXTERNALUSERS, restore_check...

mnet: restore support - XML parser knows about MNET_EXTERNALUSERS, restore_check.html warns

This may need to be reworked to move the warning to an earlier stage. In the
meantime, we have

 - at INFO parsing time we read MNET_EXTERNALUSERS into the backup info obj
 - at restore_check time, we warn the user that external users are in the
   package. If the situation looks dodgy, we say so. But don't block the
   user, as it may be a valid situation.