`cuprated` is buggy if STDIN pipe is not available
## Environment
All, [7d18098](https://github.com/Cuprate/cuprate/commit/7d18098b4ce17dea2bbd1f309bd36bcdd8f977a3).

## Bug
If STDIN is not available (e.g. docker/systemd setup), `cuprated` has buggy behavior.

## Expected behavior
Detection on when STDIN is not available.

## Steps to reproduce
1. Start `cuprated` as a normal systemd service or inside a docker container without STDIN
2. See buggy output (e.g. spamming of `help` message)