Revert "Merge pull request #57 from mandli/add-setaux-flag-mask"

This reverts commit 94998a3875ff03429e91b3e8c7addf81a3279e2f, reversing
changes made to d252f7bc04abff67d3c40ae5698a7a88f679804e.

We decided there is a better way to flag aux arrays by using aux(1,i,j)
for the flag instead of introducing a new array aux_copy_mask.

For now, revert this commit and leave code alone for 5.0.0 and then add
the flagging to 5.1, without needing to change the calling sequence of
setaux.