iscsi_create_context() calls srand() every time a new context is
generated. That practice is questionable, as the seed does not need to
change before each call to rand(). As a matter of fact, doing so defeats
the purpose of using rand() altogether. Furthermore, the current
implementation is not thread safe.
This improves ISID generation by using /dev/urandom (when available) as
a seed, and calling srand() only once. In case of errors, fallback to
using something similar to the previous implementation (albeit
thread-safe).
Signed-off-by: Felipe Franciosi <felipe@nutanix.com>