Make python arcfour_crypt more portable

[Stefan Metzmacher]

Hi Alexander,

>> here're some fixed for https://bugzilla.samba.org/show_bug.cgi?id=11699
>>
>> Crypto.Cipher.ARC4 (from python-crypto) is not available on
>> all platforms.
>>
>> We now fallback to use M2Crypto.RC4.RC4 (from python*-]m2crypto)
>> which is should be available when python-crypto is not, e.g.
>> on RHEL (at least 6?).
>>
>> Please review and push.
> It is actually a bit more complex.
> 
> RHEL6 has M2Crypto, RHEL7 has python-cryptography and we want to get rid
> of M2Crypto there. However, python-cryptography until very recently
> played badly with SELinux due to writable anonymous memory allocations,
> so we had to come with a combination like this to be able to support
> both Python 2 and Python 3:

I was wrong, python-crypto is available on debian*, ubuntu*, SLES* and
RHEL6.
RHEL7 is the only one that doesn't have it.

RHEL7 has only m2crypto.
And I see python-cryptography only on SLE12.

As my code is better than the current state, I'd suppose that
we take it as is. This fixes a bug and should be backported to 4.3
and 4.4.

We can do any further improvement later e.g. for python3 support later
in master.

metze

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