[Pipetools/coverage] Make pipes work on windows + add tests

* The authenticator field is now a XStrFixedLenField (this improves its readability when displayed using show() or show2()).

* RADIUS module update.

* The description of the compute_message_authenticator() method (RadiusAttr_Message_Authenticator class) has been edited.

* The "Response Authenticator computation" test requires the Cryptography library.

* Fix attribute names in compute_message_authenticator().

* Applied guedou's comments.

Specify field length in the Message-Authenticator attribute.

Allow sockets to be provided to send and sendp

speeds up send and sendp by preventing socket re-initialization for each send

Signed-off-by: Neale Ranns <nranns@cisco.com>

[Python 2 to 3] Fix overwritten imports (small)

[FF] Fix file format in the scapy project

[Appveyor] Add Npcap installer

[Python 2 to 3] Exceptions, loops, isinstance and haskey

[General] Update licenses & Contrib flags

Better winpcap/npcap control: scapy is now able to detect the offline
service and start it manually