[#2055] Fix makeLink while impersonating on Windows.

Problem
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We were unable to grant `SE_CREATE_SYMBOLIC_LINK` on Windows when impersonating another account.

Why we got here
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Apparently once impersonating a user token we can only adjust privileges for the current thread, and not for the whole process as impersonation acts at thread level.

Solution
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Changed `_openProcess` to fall back on opening the current thread token if unable to open the process token.

How to test
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reviewers @adiroiban 

Please take a look at the code and tests. I have trouble getting my custom user added on AIX 5.3, all the other buildslaves pass.