There are list definitions everywhere in the code, let's drop the need
for including list-t.h to declare them. The rest of the list manipulation
is huge however and not needed everywhere so using the list walking macros
still requires to include list.h.

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

This file is at the lowest level of the include tree. Its purpose is to
make sure that common types are known pretty much everywhere, particularly
in structure declarations. It will essentially cover integer types such as
uintXX_t via inttypes.h, "size_t" and "ptrdiff_t" via stddef.h, and various
type modifiers such as __maybe_unused or ALIGN() via compiler.h, compat.h
and defaults.h.

It could be enhanced later if required, for example if some macros used
to compute array sizes are needed.