gen-req overwrites an existing request without confirmation

[TinCanTech]

gen-req only checks for an existing key file.

This means that when a request is received without a key file, it is entirely possible to destroy the request accidentally. By generating another request, in the same name, the original request, tied to the correct private key, will be overwritten and will require a new private key to be distributed.

This bug has existed since at least v3.0.8, time to fix it.