REORG: memory: move the OS-level allocator to haproxy/pool-os.h

REORG: memory: move the OS-level allocator to haproxy/pool-os.h

The memory.h file is particularly complex due to the combination of
debugging options. This patch extracts the OS-level interface and
places it into a new file: pool-os.h.

Doing this also moves pool_alloc_area() and pool_free_area() out of
the #ifndef CONFIG_HAP_LOCKLESS_POOLS, making them usable from
__pool_refill_alloc(), pool_free(), pool_flush() and pool_gc() instead
of having direct calls to malloc/free there that are hard to wrap for
debugging purposes.