HPCC-9551 Roxie cannot set thread priorities to positive values

This will show up as error messages referencing pthread_setschedparam when
Roxie starts up, but will also interfere a little with the intended operation
of the UDP layer.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4563 from richardkchapman/roxie-schedule-error

HPCC-9551 Roxie cannot set thread priorities to positive values

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>