Code review comment for lp:~epics-core/epics-base/thread-pool

Rebased and made requested changes. Removed S_com_* error codes in favor of E* code from errno.h.

Also improved handling when worker thread creation fails. This was treated as an error in epicsThreadPoolCreate(). Now also in epicsJobQueue() when lazy creation of the first worker fails.

Failure to create workers after the first is not treated as an error.
This requires that user code needing to ensure that more than one worker is
running must explicitly test with epicsThreadPoolNThreads().