ENH: Teach KWSys Process basic VMS support
This achieves basic process execution on OpenVMS. We use work-arounds for different fork()/exec() behavior and a lack of select(). VMS emulates fork/exec using setjmp/longjmp to evaluate the child and parent return cases from fork. Therefore both must be invoked from the same function. Since select() works only for sockets we use the BeOS-style polling implementation. However, non-blocking reads on empty pipes cannot be distinguished easily from the last read on a closed pipe. Therefore we identify end of data by an empty read after the child terminates.
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