(G. Valadon, tickets #77 and #78)

StopAutorun can be raised in a piece of code aimed at being autorun and will
not be catched by autorun_* functions

Now, instead of creating one add-on for each new protocol, and launch each
add-on separately, you can put some load_extension() commands in your
~/scapy_startup.py and each time you run Scapy, all your extensions will
be present.

This is done either using the timeval returned by libpcap or SIOCGSTAMP ioctl for Linux native mode

The timestamp is always taken before sending the packet thus ensuring responses' timestamps are always in the future
but there is always a little overhead betwee the timestamp and the time the packet hits the wire

Used it in RandNum() and thus anything that inherit from RandNum()