Go back to using “POSIX” not “Posix”

Go back to using “POSIX” not “Posix”

In my Mon Oct 11 20:21:33 2004 +0000 commit when I standardized the
spelling of various acronyms in documentation and comments I spelled
it “Posix”.  However, it seems that “POSIX” has won out in practice
(including my own practice!), so let’s change it back to “POSIX”.