Create a new module init which contains code related to REGISTER_*
macros for initcalls. init.h is included in api.h to make init code
available to all modules.

It's a step to clean up a bit haproxy.c/global.h.

This patch fixes all the leftovers from the include cleanup campaign. There
were not that many (~400 entries in ~150 files) but it was definitely worth
doing it as it revealed a few duplicates.

Atomic ops are used about everywhere, let's make them part of the base
API by including atomic.h in api.h.

This one used to be stored into debug.h but the debug tools got larger
and require a lot of other includes, which can't use BUG_ON() anymore
because of this. It does not make sense and instead this macro should
be placed into the lower includes and given its omnipresence, the best
solution is to create a new bug.h with the few surrounding macros needed
to trigger bugs and place assertions anywhere.

Another benefit is that it won't be required to add include <debug.h>
anymore to use BUG_ON, it will automatically be covered by api.h. No
less than 32 occurrences were dropped.

The FSM_PRINTF macro was dropped since not used at all anymore (probably
since 1.6 or so).

The files currently covered by api-t.h and api.h (compat, compiler,
defaults, initcall) are now located inside haproxy/.

Let's definitely get rid of this old file.

This file includes everything that must be guaranteed to be available to
any buildable file in the project (including the contrib/ subdirs). For
now it includes <haproxy/api-t.h> so that standard integer types and
compiler macros are known, <common/initcall.h> to ease dynamic registration
of init functions, and <common/tools.h> for a few MIN/MAX macros.

version.h should probably also be added, though at the moment it doesn't
bring a great value.

All files which currently include the ones above should now switch to
haproxy/api.h or haproxy/api-t.h instead. This should also reduce build
time by having a single guard for several files at once.