ifpps: release stats on exit

The stack trace of valgrind --leak-check=full --show-reachable=yes ifpps \
-lpcd wlan0 showed that at exit members from stats are still reachable.
They have been alloced through stats_alloc, but not freed.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
ifpps: help: reduce option description to one line

Make this conform to trafgen/netsniff-ng and reduce it to 1 line
description.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>