Start and end are offset starting from netmap buf space

Conflicts:
	sys/net/netmap.h
Adding make install (leading to make headers_install) to install headers
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
Merge branch 'next' of https://code.google.com/p/netmap into next
Merge conflict
Merge branch 'next' of https://code.google.com/p/netmap into next
Merge branch 'next' of https://code.google.com/p/netmap into next
Merge branch 'next' of https://github.com/MappaM/netmap into next