smurf: Revert "Check for background (not source) pixels..."

This reverts commit a91776a7f2979f88f679f8be4b57d8ff52049c7e.

A reliable check for background pixels is not possible in smf_get_mask
since the distinction between background pixels and missing (e.g. corner)
pixels has been lost at that point because background pixels have been
replaced with bad values as a result of the AST masking.
kappa: Add option for SETQUAL to copy quality info from a template NDF

See new parameter LIKE