MinMaskGap() should care about holes as it does about pins, pads, and vias.

Otherwise, fab houses might complain because the solder mask doesn't have enough distance to holes.

The problem in detail:
ActionMinMaskGap() checks whether the hole's thickness plus the specified minimum mask gap is larger than the currently set mask value. If so, it sets the new mask to be the hole's thickness plus the specified minimum mask gap value.
ActionMinMaskGap() should check a hole's drilling diameter and base the mask calculation upon that.

This patch fixes the issue.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>