Revert last change for extract.c, due to tests failing. We can't use

Revert last change for extract.c, due to tests failing. We can't use
offset for determining whether a record stream indexing is finished
or not. Indeed filters, like ALVIS, reads large chunks at a time.
The grs.{regx,tcl} filter instead is modified to return the
"right thing", i.e. NULL data1 tree (signalling EOF / no more data).