Do not seed PRNG with uninitialised stack buffer.

Use instead some rand() filled buffer, only if OpenSSL has failed to find other source of entropy (in which case we complain at LOG_LEVEL_INFO). An example is a chroot missing ```/dev/urandom```. Even then, the sysadmin can alleviate the problem by writing random bytes to ```WORKDIR/randseed``` file.

Resolves [redmine 6468](https://dev.cfengine.com/issues/6468), [redmine 6476](https://dev.cfengine.com/issues/6476), superseeds [pull request 1887](https://github.com/cfengine/core/pull/1887), [pull request 1888](https://github.com/cfengine/core/pull/1888).
It shouldn't matter too much that the random numbers are biased: if openssl's RAND_seed() is doing its job even remotely well, it should be extracting noise from the bytes it's given, assuming that they're not marvelously pure crypto-random goodness.

As for it being a fallback, I'm not seriously suggesting the code example given; but using uint16_t instead of unsigned char would halve the number of calls to rand() that you need, for negligible effort.
```
uint16_t rand_buf[64];
for (size_t i = 0; i < sizeof(rand_buf) / sizeof(rand_buf[0]; i++)
{
    rand_buf[i] = rand();
}
RAND_seed(rand_buf, sizeof(rand_buf));
```
(and the `% 256` was pointless, anyway).