PPTP support (Enhanced GRE, PPP LCP, PPTP Control Connection)

[OneLineFix/TCPreplay] Replaced file_cache by preload_pcap

[coverage] Dot11 tests

inet_{pton,ntop}: reorganize, cleanup and improve tests

[coverage] SCTP & DHCP tests

Error out incorrect IPv4 format in IPv6 addresses in inet_pton

Enhance and fix ls()

Fix typo in tests

Fix 64 bits architectures detection

Retry .whois() test on failure

* Fix IA_PD length error

I tested this and noticed wireshark wasn't parsing the message correctly. Turns out the length is wrong.

* Update regression.uts

Fix test for DHCP6OptIA_PD

* Update length of DHCP6OptIA_PD in test.

Contrib gtp v2

Introduce FlagValue() objects to represent FlagsField() values

This PR makes it possible to check if several flags are all set, by
writting, for example, `if pkt[TCP].flags.SA:`.

It only works for single letter flags (e.g., will work for TCP().flags
but not for IP().flags), and only when getting a value.

Thanks @guedou for suggesting that.