MINOR: debug/memstats: permit to pass the size to free()

MINOR: debug/memstats: permit to pass the size to free()

Right now the free() call is not intercepted since all this is done
using macros and that would break a lot of stuff. Instead a __free()
macro was provided but never used. In addition it used to only report
a zero size, which is not very convenient.

With this patch comes a better solution. Instead it provides a new
will_free() macro that can be prepended before a call to free(). It
only keeps the counters up to date, and also supports being passed a
size. The pool_free_area() command now uses it, which finally allows
the stats to look correct:

  pool-os.h:38   MALLOC  size:   5802127832  calls:   3868044  size/call:   1500
  pool-os.h:47     FREE  size:   5800041576  calls:   3867444  size/call:   1499

The few other places directly calling free() could now be instrumented to
use this and to pass the correct sizeof() when known.