split: adjust recent --number changes

split: adjust recent --number changes

* src/split.c (lines_rr): Reinstate the conditional
setting of the WROTE boolean, as otherwise split -n r/1 would
consume all input when all --filter commands are stopped.
There was a test in place to check for this, but
it was incorrect as detailed below.
(input_file_size): Immediately disallow --number with
non seekable inputs, as such an invocation is not currently
generally supported and will fail as the data overflows
the internal buffer.
* tests/split/l-chunk.sh: Adjust to again disallow -n /dev/zero.
Also change all '&& fail=1' checks to use the 'returns_ 1' form.
* tests/split/filter.sh: Change the no longer supported /dev/zero
case to a regular $OFF_T_MAX file (supported on XFS for example).
Also fix the timeout(1) commands so they're not subject to
pipefail issues.