Revert "Merge pull request #93 from mandli/add-setaux-flag-mask"

This reverts commit 71e910c8225d515caf19b4725e1305b006ceb7a2, reversing
changes made to ab5e7b17c310acba48da864e117c73ec8e626b2a.

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.