This should avoid random unrelated crashes.

NTPControl: rename field 'error' in 'err' in order to have the same name for the error field in NTP Control messages and NTP Private messages.

Use the XStrFixedLenField class which is now defined in scapy/fields.py + Fix typo.

* Improved main.py tests

* Small fixes

* 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.

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.

It is now possible to write `pkt[TCP].flags.S = True`.

This makes it possible to write `if pkt[TCP].flags.S: [...]`.