Prevent very large #option('outputLimit') causing grief to Dali.

The default workunit(dali) result limit is 10MB and for good reason, anything genuinely large, should be written to a disk output file and not fill memory on Dali.

The #option('outputLimit', <MB>) is there where it is awkward to avoid, sometimes that's because the workunit results are created internally by the codegen - even in those cases, the limit should not just be upped, it should be regarded as a hint to look at the ECL and recode it where possible.

To prevent GB's of data being written by misuse of #option('outputLimit') - impose an upper hard limit, which it cannot be set above.