rename Avoiding Linuxisms page, and add a blurb on POSIX

[jgh]

Summary

Generally, I didn't like the name of this page. I didn't like it for a couple of reasons, but namely why sling mud at others. The other reason was that Linux was only noted three times in the entire section, yet we named the entire section about avoiding Linuxisms.

This is the combined work of wblock and jgh

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