wesnothd: Only reset the commands FIFO stream when reloading if needed

May or may not fix the issue noted in commit
62eb55a5a712b3e2157cf7fc0bb12873c4f0a574 regarding the SIGHUP handler,
which is in charge of scheduling load_config() to run. Rather
unfortunately, I'm unable to reproduce the issue on my own machine, so
I'll have to take this to production and test it live.