For "individually different but deterministic" runs specifying Zeek's
seed as an environment variable eases setups as one can avoid creating
extra seed files for each of the individual processes.
It is an error to specify the new ZEEK_SEED_VALUES variable together
with the existing ZEEK_SEED_FILE and -G. ZEEK_SEED takes precedence over
deterministic mode (-D) like ZEEK_SEED_FILE does today already.
