MEDIUM: debug: improve DEBUG_MEM_STATS to also report pool alloc/free

MEDIUM: debug: improve DEBUG_MEM_STATS to also report pool alloc/free

Sometimes using "debug dev memstats" can be frustrating because all
pool allocations are reported through pool-os.h and that's all.

But in practice there's nothing wrong with also intercepting pool_alloc,
pool_free and pool_zalloc and report their call counts and locations,
so that's what this patch does. It only uses an alternate set of macroes
for these 3 calls when DEBUG_MEM_STATS is defined. The outputs are
reported as P_ALLOC (for both pool_malloc() and pool_zalloc()) and
P_FREE (for pool_free()).