s6-cleanfifodir cleans up a fifodir.
s6-cleanfifodir cleans up fifodir, that must belong to the current user. That means it removes all stale FIFOs in fifodir.
In normal use, it is not necessary to call s6-cleanfifodir. However, stale FIFOs can be left by s6-ftrigrd processes that were violently killed, so it's good practice to regularly clean up fifodirs.