Enhance ENQC support and provide greater visibility/alerting into the number of ECBs waiting on events

1) There is no means currently for an application to know how many ECBs are waiting on a resource. This means that applications have only two processing options for ENQC 1) if they get the resource
or 2) if they don't get the resource.

Additional processing options are needed that allow an application to take an alternative processing path if the resource is currently held and the number of ECBs waiting on the resource exceeds an acceptable value. This would allow an application to avoid significant backlogs if an alternative processing path were available to it. It would be up to the application to determine what the “acceptable” number of waiters was, but enhancements to the ENQC API are needed such that it :
a) can optionally return the number of waiters if a resource is currently held
b) accepts a MAXWAITER and MAXWAITERERR parameter so that the ECB is queued on a held resource only if the number of waiting ECBs is less than the specified MAXWAITER value. The MAXWAITERERR parm would provide a branch location to indicate that the resource was held because it was already held and the number of waiting ECBs was greater than the MAXWAITER value specified.

2) The count of waiting ECBs for any given type of event in the event table has to be determined by counting the number of chained SWBs. A count of waiting ECBs should be added to the event table item. This would allow
a) fast support for the ENQC enhancement in item 1 above
b) allow ZDEVN to immediately access the waiter count without chasing SWB chains
c) provide the infrastructure for alerting based on number of waiters (see point 3 below).

3) We need to be able to alert on event table waiters exceeding a user defined value, much like we can currently alert on RHT waiters. The alert would not take any action, it would be a warning mechanism of contention build up. User exits in the support might be useful, but the default action for events that had long queues of waiters should be only to report.