More than 1 vlan support

[yogch]

Hi,
Can libnids support more than one vlan?
For now it doesn't.
In src/libnids.c:253.
It's harcodedly checks for next header as 'ip' or 'vlan'. If it's vlan it assumes the header after the vlan header is the ip header. It doesn't check whether there's another vlan after the first vlan.