Fixed some typos and bad whitespaces.

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

  - this feature allows conversion of large ranges where
    all entries coalesce to the same representation without bloating
    a dict. For instance, 0x0 to 0xffff all display as "toto" except
    a few odds 0x2 which are displayed as "tutu".
    Before this feature, you needed to write
    enum = {x: 'toto' for x in xrange(0, 0xffff)
    enum[2] = 'tutu'
    XShortEnumField('test', 0, enum)