10573 define TASKQID_INVALID as (taskq_id)0
illumos/illumos-gate@fc8ae2ec4282de7ec96f48e11078345f3dc0ac3d
https://github.com/illumos/illumos-gate/commit/fc8ae2ec4282de7ec96f48e11078345f3dc0ac3d
https://www.illumos.org/issues/10573
We do have taskqid_t taskq_dispatch() and the result of this function is compared with 0, NULL and combined with type casts. Define TASKQID_INVALID (taskq_id)0 and use TASKQID_INVALID everywhere.
Author: Toomas Soome <tsoome@me.com>