i.rectify and masks

A couple of questions:

1) How is spatial interpolation peformed by i.rectify? Cubic
    convolution? Nearest neighbor resampling?

2) How are GRASS users creating polygonal masks now that
    mapmask is not available in version 4.0? I want to define
    circular-shaped catchments around sites and compare only
    the land use/land cover within these areas.

Any ideas would be greatly appreciated.

Cliff Behrens