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).
I would be inclined to use `(pid * start_time) ^ (fqdn_len * now)` to get a better mix of operations with less overflow.
Overflow throws away entropy that was in the data being combined; and multiplying two times does lots of it.  Multiplying each time by something else, then XORing the results does less overflow while combining the entropies of the parts.