postmaster.c and random keys/salts

The following is used to key a channel (it appears to be used in the
AUTH_REQ_MD5 in fe-auth.c).

Four bytes is a tad bit small, and time based does not leave much to
the imagination. Also, it looks like its susceptible to VM roillbacks.

**********

static void
RandomSalt(char *md5Salt)
{
    long        rand;

    /*
     * We use % 255, sacrificing one possible byte value, so as to ensure that
     * all bits of the random() value participate in the result. While at it,
     * add one to avoid generating any null bytes.
     */
    rand = PostmasterRandom();
    md5Salt[0] = (rand % 255) + 1;
    rand = PostmasterRandom();
    md5Salt[1] = (rand % 255) + 1;
    rand = PostmasterRandom();
    md5Salt[2] = (rand % 255) + 1;
    rand = PostmasterRandom();
    md5Salt[3] = (rand % 255) + 1;
}

/*
 * PostmasterRandom
 */
static long
PostmasterRandom(void)
{
    /*
     * Select a random seed at the time of first receiving a request.
     */
    if (random_seed == 0)
    {
        do
        {
            struct timeval random_stop_time;

            gettimeofday(&random_stop_time, NULL);

            /*
             * We are not sure how much precision is in tv_usec, so we swap
             * the high and low 16 bits of 'random_stop_time' and XOR them
             * with 'random_start_time'. On the off chance that the result is
             * 0, we loop until it isn't.
             */
            random_seed = random_start_time.tv_usec ^
                ((random_stop_time.tv_usec << 16) |
                 ((random_stop_time.tv_usec >> 16) & 0xffff));
        }
        while (random_seed == 0);

        srandom(random_seed);
    }

    return random();
}