nfacct: adding quota capabilities

The accounting framework now supports quota at the packet and byte
level.  The tool is simply enhanced with two optional arguments to
specify the whether accounting for byte of packet and the limit
associated with each.  Also adding a monitor mode that listens for
quota attainment notification.

Examples:

/* create an accounting object that isn't associated to a quota */
$ nfacct add first_no_quota

/* create a quota object with byte count limited to 50 byte */
$ nfacct add second_quota byte 50

/* create a quota object with packet count limited to 5 */
$ nfacct add third_quota packet 5

From there the accounting objects can be used in iptables the same
way as they did before:

/* limit the number of icmp packets allowed through the OUTPUT chain */
$ iptables -I OUTPUT -p icmp -m nfacct --nfacct-name third_quota -j REJECT

/* listening for quota attainment notification */
$ nfacct monitor

Everything else works the same way.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
nfacct: simplify nfacct_cmd_monitor

Simplify the code that was added in ba16753 ("nfacct: adding quota
capabilities").

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>