Start and end are offset starting from netmap buf space

Conflicts:
	sys/net/netmap.h
Adding global extra buffer allocation (not relied to any interface).

Conflicts:
	sys/net/netmap.h
Support for multiple NIC attachment (in test)
Adding buf_end for extra buffer request
Start and end are offset starting from netmap buf space
Adapting to header change for mem
Update README
Support for echo in multithread
Idem