kernel: Fix some void functions returning the results of void functions.

It was found with -pedantic, since -Wreturn-type will not warn about this
as long as the calling function has a return type of void, too.
pppd.8: conversion to mdoc(7)

Major overhaul of a well-written manual page.  The content has
been shuffled around and subtly cleaned where appropriate.

Suggested-by: swildner