RAWNAT: add extension's kernel and userspace modules

RAWNAT provides stateless 1:1 network address translation.

extensions/libxt_RAWSNAT.man

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+The \fBRAWSNAT\fR and \fBRAWDNAT\fP targets provide stateless network address
+translation.
+.PP
+The \fBRAWSNAT\fR target will rewrite the source address in the IP header, much
+like the \fBNETMAP\fP target. \fBRAWSNAT\fP (and \fBRAWDNAT\fP) may only be
+used in the \fBraw\fP or \fBrawpost\fP tables, but can be used in all chains,
+which makes it possible to change the source address either when the packet
+enters the machine or when it leaves it. The reason for this table constraint
+is that RAWNAT must happen outside of connection tracking.
+.TP
+\fB--to-source\fR \fIaddr\fR[\fB/\fR\fImask\fR]
+Network address to map to. The resulting address will be constructed the
+following way: All 'one' bits in the \fImask\fR are filled in from the new
+\fIaddress\fR. All bits that are zero in the mask are filled in from the
+original address.
+.PP
+As an example, changing the destination for packets forwarded from an internal
+LAN to the internet:
+.IP
+-t raw -A PREROUTING -i lan0 -d 212.201.100.135 -j RAWDNAT --to-destination 199.181.132.250
+-t rawpost -A POSTROUTING -o lan0 -s 199.181.132.250 -j RAWSNAT --to-source 212.201.100.135
+.PP
+Note that changing addresses may influence the route selection! Specifically,
+it statically NATs packets, not connections, like the normal DNAT/SNAT targets
+would do. Also note that it can transform already-NATed connections -- as said,
+it is completely external to Netfilter's connection tracking/NAT.
+.PP
+If the machine itself generates packets that are to be rawnat'ed, you need a
+rule in the OUTPUT chain instead, just like you would with the stateful NAT
+targets.
+.PP
+It may be necessary that in doing so, you also need an extra RAWSNAT rule, to
+override the automatic source address selection that the routing code does
+before passing packets to iptables. If the connecting socket has not been
+explicitly bound to an address, as is the common mode of operation, the address
+that will be chosen is the primary address of the device through which the
+packet would be routed with its initial destination address - the address as
+seen before any RAWNAT takes place.