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).
You're throwing away three quarters of the entropy you're generating, with that `% 256`.
How about using an `unsigned int rand_buf[128 / sizeof(int)]`, or just `[128]`, and using all of it ?