System #include overhaul

System #include overhaul

Reorganize `#include <>`s in accordance with the IEEE Std 1003.1,
the Linux man pages (which do a pretty good job explaining portability
nuances), the documentation of the dependencies and some common sense.
(Since it seems some of this stuff is undefined.)

The algorithm still induces some unnecessary includes. (eg. the `NULL`
symbol induces `stddef.h`, `string.h`, `stdlib.h`, `stdio.h`, `unistd.h`
AND `locale.h`, because the standard states it should be defined in all
of them.) I don't think this is a problem for now; I'll optimize it
later.