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Add in randomness based on Linux's thread id (gettid) to avoid race
conditions when two threads try to generate uuid's at the same time.
This shouldn't be an issue if /dev/urandom has proper locking and is
present, so this is just a failsafe.

Addresses SourceForge Bug: #1529672
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>