We only have a certain granularity of how our headers in "shared/nm-utils"
can be used independently.

For example, it's not supported to use "nm-macros-internal.h" without
"gsystem-local-alloc.h". Likewise, you cannot use "nm-glib.h" directly,
you always get it together with "nm-macros-internal.h".

This is, we don't support to use certain headers entirely independently,
because usually you anyway want to use them together.

As such, no longer support "gsystem-local-alloc.h", but merge the
remainder into "nm-macros-internal.h". There is really no reason
to support arbitrary flexibility of including individual bits. You
want cleanup-macros? Include "nm-macros-internal.h".

Merge the headers.