MINOR: haproxy: prepare to move limits-related code

MINOR: haproxy: prepare to move limits-related code

This patch is done in order to prepare the move of handlers to compute and to
check process related limits as maxconn, maxsock, maxpipes.

So, these handlers become no longer static due to the future move.

We add the handlers declarations in limits.h in this patch as well, in order to
keep the next patch, dedicated to code replacement, without any additional
modifications.

Such split also assures that this patch can be compiled separately from the
next one, where we moving the handlers. This  is important in case of
git-bisect.