ipset: add "inner" flag support

This series of 5 patches implements "inner" flag option in the set iptables match, allowing matching based on the properties (source/destination IP address, protocol, port and so on) of the original (inner) connection in the event of the following ICMP[v4,v6] messages:

ICMPv4 destination-unreachable (code 3);
ICMPv4 source-quench (code 4);
ICMPv4 time-exceeded (code 11);
ICMPv6 destination-unreachable (code 1);
ICMPv6 packet-too-big (code 2);
ICMPv6 time-exceeded (code 3);

Revision history:

v1 * initial revision
v2 * redundant code removed;
* added a new header file (ip_set_icmp.h) with 2 inline functions,

     allowing access to the internal icmp header properties;
   * removed ip[46]inneraddr[ptr]functions as they are no longer needed
   * added new ipv[46]addr[ptr] and ip_set_get*port functions, the old
     functions are still preserved for backwards compatibility;

Dash Four (5):
iptables: bugfix: prevent wrong syntax being accepted by the set match
ipset: add "inner" flag implementation
ipset: add set match "inner" flag support
iptables: add set match "inner" flag support
iptables (userspace): add set match "inner" flag support