Add dbg_before() and dbg_after()

Other two debug utilities to compute filter rate.

Usage is:

dbg_before(); // counts passages from this point

if(..) // complex code stuff you want to audit
  return/continue
if(...)
  .....

dbg_after(); // counts passages from this point

Then somewhere in the code, normally in poll() add
dbg_print_hit_rate() and you will see the filter rate
of your code under auditing.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>