libuuid: try to use getrandom() or getentropy() if available

libuuid: try to use getrandom() or getentropy() if available

If getrandom() or getentropy() is available, use these interfaces in
favor of opening /dev/[u]random.  This avoids a potential TSAN problem
that could potentially cause a fd leak when trying to open
/dev/urandom.  (Which is not a disaster, but these interfaces are more
foolproof and avoids needing to open a file descriptor in a library,
which is a good thing.)

Addresses-Google-Bug: #198050608
Signed-off-by: Theodore Ts'o <tytso@mit.edu>