Fix ipv4 header initialization.
Fix ipv4 header initialization.

The version/header length/TOS field is 16 not 32 bits wide.
Also add a self-test.

dpino/snabbswitch,heryii/snabb,eugeneia/snabb,alexandergall/snabbswitch,wingo/snabb,plajjan/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,dwdm/snabbswitch,andywingo/snabbswitch,andywingo/snabbswitch,wingo/snabb,wingo/snabbswitch,alexandergall/snabbswitch,wingo/snabb,mixflowtech/logsensor,hb9cwp/snabbswitch,kbara/snabb,kbara/snabb,pirate/snabbswitch,kbara/snabb,snabbco/snabb,alexandergall/snabbswitch,dpino/snabb,wingo/snabbswitch,heryii/snabb,aperezdc/snabbswitch,dpino/snabb,dpino/snabb,snabbco/snabb,plajjan/snabbswitch,Igalia/snabb,heryii/snabb,dpino/snabb,wingo/snabbswitch,eugeneia/snabbswitch,wingo/snabb,hb9cwp/snabbswitch,justincormack/snabbswitch,SnabbCo/snabbswitch,kbara/snabb,alexandergall/snabbswitch,lukego/snabb,Igalia/snabbswitch,plajjan/snabbswitch,Igalia/snabb,Igalia/snabb,eugeneia/snabb,eugeneia/snabb,alexandergall/snabbswitch,pirate/snabbswitch,Igalia/snabbswitch,alexandergall/snabbswitch,justincormack/snabbswitch,andywingo/snabbswitch,dpino/snabbswitch,alexandergall/snabbswitch,snabbnfv-goodies/snabbswitch,Igalia/snabbswitch,snabbnfv-goodies/snabbswitch,snabbnfv-goodies/snabbswitch,justincormack/snabbswitch,SnabbCo/snabbswitch,dwdm/snabbswitch,snabbco/snabb,snabbco/snabb,dwdm/snabbswitch,justincormack/snabbswitch,mixflowtech/logsensor,wingo/snabb,eugeneia/snabbswitch,plajjan/snabbswitch,dpino/snabbswitch,SnabbCo/snabbswitch,Igalia/snabb,andywingo/snabbswitch,lukego/snabb,aperezdc/snabbswitch,mixflowtech/logsensor,heryii/snabb,dpino/snabb,hb9cwp/snabbswitch,Igalia/snabb,SnabbCo/snabbswitch,dpino/snabb,pirate/snabbswitch,lukego/snabbswitch,heryii/snabb,mixflowtech/logsensor,snabbco/snabb,Igalia/snabbswitch,snabbco/snabb,lukego/snabbswitch,lukego/snabb,Igalia/snabb,lukego/snabbswitch,snabbnfv-goodies/snabbswitch,kbara/snabb,Igalia/snabb,eugeneia/snabb,snabbco/snabb,dpino/snabb,heryii/snabb,aperezdc/snabbswitch,lukego/snabbswitch,dpino/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,eugeneia/snabbswitch,Igalia/snabb,mixflowtech/logsensor,kbara/snabb,aperezdc/snabbswitch,eugeneia/snabbswitch,eugeneia/snabb,wingo/snabbswitch,eugeneia/snabb,hb9cwp/snabbswitch,lukego/snabb,eugeneia/snabb,wingo/snabb