[MERGE] Cleanup and test Lock objects

[John Arbash Meinel]

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The attached patch cleans up the bzrlib/lock.py file a bit, and adds
direct test for our Lock support.

I still have more to do to make locks properly exclusive on Linux. But
because that is a behavior change, I wanted to submit this first.

This also implements Windows locks using ctypes if pywin32 is not
available. It turns out that msvcrt locks don't support creating a
shared lock, so we can no longer use them.

Which means that we now have a strict dependency on either pywin32 or
ctypes for Windows. I don't think this is terrible, though, because
we've had a soft one for a long time. And as I understand it, Alexander
has been shipping a installer with them for a long time.

My next step is to enable the 2 tests that ensure you cannot take a read
lock on the same file that you have a write lock on. Which I know fails
with fcntl locks, and I expect that once I fix that, we will start
seeing lots of LockErrors as part of the rest of our code.

John
=:->
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