Return the system's entropy bit count, or -1 if unknown.

def system_entropy():
        """Return the system's entropy bit count, or -1 if unknown."""
        arg = ['cat', '/proc/sys/kernel/random/entropy_avail']
        proc = Popen(arg, stdout=PIPE, stderr=DEVNULL)
        response = proc.communicate()[0]
        return int(response) if response else -1