Implement CoCreateGuid using uuid_generate on Unix

Implement CoCreateGuid using uuid_generate on Unix

We were generating GUIDs on Unix as random numbers. But that is not
correct since GUIDs have defined structure with bits having specific
meanings. For example, there are four bits that represent the type of
the GUID, which means whether the guid is randomly generated,
name based on SHA1, time based etc.
This change changes the CoCreateGuid to use the uuid_generate function
from the uuid library that should generate well-formed GUIDs on
Linux and OSX and the uuid_create on FreeBSD.