DEBUG: init: add a way to register functions for unit tests

DEBUG: init: add a way to register functions for unit tests

Doing unit tests with haproxy was always a bit difficult, some of the
function you want to test would depend on the buffer or trash buffer
initialisation of HAProxy, so building a separate main() for them is
quite hard.

This patch adds a way to register a function that can be called with the
"-U" parameter on the command line, will be executed just after
step_init_1() and will exit the process with its return value as an exit
code.

When using the -U option, every keywords after this option is passed to
the callback and could be used as a parameter, letting the capability to
handle complex arguments if required by the test.

HAProxy need to be built with DEBUG_UNIT to activate this feature.