ipc: fix IO::FDPass use with a worker limit of 1

ipc: fix IO::FDPass use with a worker limit of 1

IO::FDPass is our last choice for implementing the workqueue
because its lack of atomicity makes it impossible to guarantee
all requests of a single group hit a single worker out of many.

So the only way to use IO::FDPass for workqueues it to only have
a single worker.  A single worker still buys us a small amount
of parallelism because of the parent process.