packet_filter.conntrack: Allow non-IP packet arguments
packet_filter.conntrack: Allow non-IP packet arguments

Previously it was an error to pass non-IP packets to check(). Now
these are treated as not belonging to any session.

Added a brief API summary in comments.

snabbco/snabb,virtualopensystems/snabbswitch,pirate/snabbswitch,xdel/snabbswitch,wingo/snabb,snabbco/snabb,hb9cwp/snabbswitch,virtualopensystems/snabbswitch,dpino/snabb,hb9cwp/snabbswitch,kbara/snabb,Igalia/snabb,andywingo/snabbswitch,snabbco/snabb,eugeneia/snabbswitch,hb9cwp/snabbswitch,Igalia/snabb,heryii/snabb,Igalia/snabbswitch,snabbco/snabb,lukego/snabb,fhanik/snabbswitch,aperezdc/snabbswitch,pirate/snabbswitch,SnabbCo/snabbswitch,pavel-odintsov/snabbswitch,virtualopensystems/snabbswitch,snabbco/snabb,dpino/snabb,kbara/snabb,heryii/snabb,pirate/snabbswitch,lukego/snabb,wingo/snabbswitch,SnabbCo/snabbswitch,snabbnfv-goodies/snabbswitch,Igalia/snabb,dpino/snabb,alexandergall/snabbswitch,mixflowtech/logsensor,alexandergall/snabbswitch,dpino/snabb,eugeneia/snabb,lukego/snabb,lukego/snabbswitch,eugeneia/snabbswitch,alexandergall/snabbswitch,snabbnfv-goodies/snabbswitch,lukego/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,dwdm/snabbswitch,Igalia/snabbswitch,SnabbCo/snabbswitch,dpino/snabbswitch,lukego/snabb,alexandergall/snabbswitch,wingo/snabb,dwdm/snabbswitch,wingo/snabbswitch,kbara/snabb,aperezdc/snabbswitch,mixflowtech/logsensor,wingo/snabb,dpino/snabbswitch,wingo/snabb,mixflowtech/logsensor,dpino/snabbswitch,alexandergall/snabbswitch,snabbnfv-goodies/snabbswitch,eugeneia/snabb,eugeneia/snabb,fhanik/snabbswitch,pavel-odintsov/snabbswitch,eugeneia/snabb,eugeneia/snabb,lukego/snabbswitch,Igalia/snabbswitch,snabbco/snabb,wingo/snabb,plajjan/snabbswitch,Igalia/snabb,kbara/snabb,Igalia/snabb,snabbco/snabb,kbara/snabb,Igalia/snabbswitch,wingo/snabbswitch,heryii/snabb,alexandergall/snabbswitch,aperezdc/snabbswitch,snabbco/snabb,javierguerragiraldez/snabbswitch,justincormack/snabbswitch,snabbnfv-goodies/snabbswitch,Igalia/snabb,aperezdc/snabbswitch,fhanik/snabbswitch,dpino/snabb,andywingo/snabbswitch,xdel/snabbswitch,plajjan/snabbswitch,javierguerragiraldez/snabbswitch,eugeneia/snabb,eugeneia/snabbswitch,javierguerragiraldez/snabbswitch,kellabyte/snabbswitch,dpino/snabb,eugeneia/snabbswitch,heryii/snabb,andywingo/snabbswitch,plajjan/snabbswitch,justincormack/snabbswitch,heryii/snabb,plajjan/snabbswitch,justincormack/snabbswitch,mixflowtech/logsensor,Igalia/snabb,lukego/snabbswitch,dwdm/snabbswitch,SnabbCo/snabbswitch,mixflowtech/logsensor,kellabyte/snabbswitch,andywingo/snabbswitch,dpino/snabbswitch,kellabyte/snabbswitch,Igalia/snabbswitch,alexandergall/snabbswitch,xdel/snabbswitch,eugeneia/snabb,wingo/snabb,hb9cwp/snabbswitch,wingo/snabbswitch,pavel-odintsov/snabbswitch,justincormack/snabbswitch,kbara/snabb,Igalia/snabb,dpino/snabb,heryii/snabb