This is of course trivial. However, we use this macro at several places
as and index into an array of length 2, to lookup either the IPv4 or
IPv6 element. As such, this MUST return 0 or 1. This promise is what the
macro should convey.
There is the team of nm_auto*, nm_clear_pointer() and nm_steal_pointer().
These goes nicely together so that an autovariable owns a resource,
to free it (clear) and to transfer ownership (steal).
We have these also in glib, but we certainly also need it if we don't
have glib because that very much goes together with nm_auto*. Add it.
For a long time already we build with gnu11/C11. We thus rely on having
<stdbool.h> around, which is C99. Use it.
Also fix "nm-std-aux.h" to not use TRUE/FALSE glib defines.
We redefine _G_BOOLEAN_EXPR(), so let it use NM_BOOLEAN_EXPR().
Also, we use G_LIKELY() (and thus NM_BOOLEAN_EXPR()) inside nm_assert(),
and we use nm_assert() in some macros. To be able to nest nm_assert()
calls, we need to create unique variable names for NM_BOOLEAN_EXPR().
Having assertion macros that are disabled by default, is not
only useful for our glib code, but should also be available
for nm-std-aux. Move the macros.
