drm/i915: Use Linux atomic types and functions
usb4bsd: make u4b compile with USB_DEBUG
<sys/cdefs.h>: Add a __heedresult macro to warn of discarded return values.

It's an alias for __attribute__((__warn_unused_result__)) and can be used
to mark functions whose return value must never be ignored by the calling
code.
colldef(1)/mklocale(1): Define __heedresult when BOOTSTRAPPING.

These two utilities are bootstrap tools, and due to already existing
surgery take part of their includes from the src/, so as a consequence
we have to define __heedresult too in the BOOTSTRAPPING case.

This is actually the sort of issues which the bootstrap-tools concept
is meant to protect us against. :-(
Mark malloc() and various similar functions with __heedresult.

calloc(), malloc(), realloc(), reallocf(), aligned_alloc(),
contigmalloc(), kmalloc_debug(), krealloc_debug(), kstrdup_debug(),
kmalloc(), krealloc(), kstrdup(), kmalloc_cachealign().

This will generate a warnings/errors when their return value is ignored.