fix typo.
fix warnings
fix the snaplen of pcap_open_live()
Construct VLAN tag from auxdata.
enable non-blocking mode when using pcap library
fix warnings

`rake run_unitttests` stops because of -Werror
add comment in unused unittest
Merge branches 'fix_unittests' and 'fix_unittests' of https://github.com/iHiroakiKawai/trema-edge into fix_unittests