MINOR: haproxy: add a registration for post-check functions

MINOR: haproxy: add a registration for post-check functions

There's a significant amount of late initialization calls which are
performed after the point where we exit in check mode. These calls
are used to allocate resource and perform certain slow operations.
Let's have a way to register some functions which need to be called
there instead of having this multitude of #ifdef in the init path.