We already have "nm-utils*.h" and "NetworkManagerUtils.h" headers. Rename
"include/nm-utils-internal.h" to "nm-macros-internal.h". I think that
name is better, because this file is header-only, internal, and
repository-wide.

Also, it will never contain non-header-only declarations because
there is no backing object file under "include/".
It will only contain macros and inline functions.