Improve PPP Authentication support

DNS TSIG Ressource Record

GTPHeader class.

modbus: fix len handling for function code 0x2b + tests

Flags: use str(obj) rather than obj.flagrepr() (more "Pythonic")

Add a mysummary() method to the EAP class.

Don't call randval() on unused ConditionalField

[Speed up] Use passive form to select.select + Fix pipetools.uts

* ikev2 contrib fixup

* allow unknown transform_type and transform_id

* Simplify Authentication payload type

* CERT payload: add x509Cert and x509CRL attributes

* First re-indent

* Fixed ID not being a number

* Add some small IKEv2 tests

* Replaced tabs by spaces

* Separate classes by type

* New tests- small fixes

Tacacs+ support

* TLS module major overhaul

* Clean TLS directory

* TLS 1.3 packets and basic client automaton (+ session fixes)

* Fix multiple key share entries

* Convergence py3

* Support features from cryptography v2.0

* Validate CertificateVerify signatures

* Small fix on TLS signature parsing

* Small fixes in TLS module

* Enhance Encrypted PMS support

* Convergence py3 - print

* Revert useless changes

* Strengthen crypto lib validation

checksum() example added in doc