better dampening stats

Patch to dampening.c/.h which keeps track of the number of dampened IP
addresses, and reports it along with the rest of the stats.

Stats output before patch:
Apr  8 22:17:14 ns1b named[33694]: Stats for #0: queries 283/177/0:
lock=0.001802, search=0.001973, update=0.001790, add=0.000000

After patch:
Apr  8 22:17:14 ns1b named[33694]: Stats for #0: dampened: 30 - queries
283/177/0: lock=0.001802, search=0.001973, update=0.001790, add=0.000000