Bug fixes, MFile & ProcPool enhancements

This release fixes a couple bugs. It slightly enhances cligen/mfile.MFile.

Usage by @Vindaar helped to motivate hardening procpool.nim interfaces to avoid deadlock from insufficient statistical regularity and one breaking change (to avoid same-fd-as-parent footguns). Also, silence nCPU-way repetitions of Control-C messages (but provide a way to get them back if users want).

Also, remove need for (not so perf helpful anyway) || iterator in examples/dups.nim.