This patch adds a new match called 'hashlimit'.
The idea is to have something like 'limit', but either per
destination-ip or per (destip,destport) tuple.

It gives you the ability to express
.IP
 '1000 packets per second for every host in 192.168.0.0/16'
.IP
 '100 packets per second for every service of 192.168.1.1'
.P
with a single iptables rule.
.TP
.BI "--hashlimit " "rate"
A rate just like the limit match
.TP
.BI "--hashlimit-burst " "num"
Burst value, just like limit match
.TP
.BI "--hashlimit-mode " "destip | destip-destport"
Limit per IP or per port
.TP
.BI "--hashlimit-name " "foo"
The name for the /proc/net/ipt_hashlimit/foo entry
.TP
.BI "--hashlimit-htable-size " "num"
The number of buckets of the hash table
.TP
.BI "--hashlimit-htable-max " "num"
Maximum entries in the hash
.TP
.BI "--hashlimit-htable-expire " "num"
After how many miliseconds do hash entries expire
.TP
.BI "--hashlimit-htable-gcinterval " "num"
How many miliseconds between garbage collection intervals
