smurf: Allow AST to be skipped on all iterations

Setting a negative value for AST.SKIP will result in no more than
-AST.SKIP iterations being performed, and the AST model will be skipped
on all iterations. In this case, AST.SKIP takes the place of NUMITER.
smurf: Fix bug that caused segfault when copying detname or ocsconfig

See Starlink issue #28