Added support for MPLS

The FlowMon probe is now able to operate on links with MPLS, monitoring MPLS-encapsulated packets along with plain IPv4 and IPv6.

Latest versions of programs for the Header Field Extractor nanoprocessor recognize the shim headers of MPLS packets and collect the flow-related information from the embedded IP packet. Although NetFlow v9 specifications defines fields for MPLS labels, they are not supported yet by the current version.

See http://www.flowmon.org/flowmon-probe/userdocs/mpls for installation instructions.