HPCC-9000 Variable length record display issues

Records with no maxlength but with records > 16k in size may display garbage
in ECLWatch.

This fix detects such cases and displays an error instead, and at the same
time raises the limit to 64k.

A full fix will require more work.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4153 from richardkchapman/file-display-large

HPCC-9000 Variable length record display issues

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>