netmix: remove non-exisiting targets from Makefile
netmix: consider MAC header in MTU for TAP devices

When using a TAP device, the maximum packet size includes the MAC
header, which is not given when reading the MTU for the virtual
interface, leading to truncated packets. Fix this by adding ETH_HLEN
bytes to the read MTU, if virtual interface is of type TAP.
netmix: remove unused rlnc_hdr_mux.hpp
netmix: remove arguments.hpp

src/plain_relay.cpp was the last user of arguments.hpp and did not
actually use it, so purge it.
Merge branch 'master' of https://github.com/hundeboll/netmix