MINOR: threads: Add mechanism to register per-thread init/deinit functions

MINOR: threads: Add mechanism to register per-thread init/deinit functions

hap_register_per_thread_init and hap_register_per_thread_deinit functions has
been added to register functions to do, for each thread, respectively, some
initialization and deinitialization. These functions are added in the global
lists per_thread_init_list and per_thread_deinit_list.

These functions are called only when HAProxy is started with more than 1 thread
(global.nbthread > 1).