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Tweak ppoll() to include 1003.1-2024 visibility

Description

Tweak ppoll() to include 1003.1-2024 visibility

Note in the manpage that the 2024 edition finally added ppoll(), and
also add the appropriate declarations for the correct versions of
_POSIX_C_SOURCE.

Differential Revision: https://reviews.freebsd.org/D48043