Use generic L3RawSocket rather than OS specific L3PacketSocket

Experimental: Seems to work in tests, but has not yet been tested
on any platform other than Linux. Solves the problem reported in
https://github.com/TheTorProject/ooni-probe/issues/214
Use the non raw scapy super socket when using Mac OS X
Merge branch 'fix/use_generic_L3socket'

* fix/use_generic_L3socket:
  Use the non raw scapy super socket when using Mac OS X
  Use generic L3RawSocket rather than OS specific L3PacketSocket
Merge branch 'master' into fix/misc

* master:
  Use the non raw scapy super socket when using Mac OS X
  Use generic L3RawSocket rather than OS specific L3PacketSocket

Conflicts:
	ooni/utils/txscapy.py