Add support for multiple VLANs per ifIndex

sFlow and Netflow v9/v10 may provide VLAN information in flows, so
one can distinguish between different logical links on the same
physical connection (e.g. transit via private VLAN over IXP).
Corrected paramters to handleflow()
Avoid warnings about undefined values
Merge pull request #31 from green525/master

Support VLANs to distinguish different logical links