fix: do not block in accept

[c-cube]

cc @vphantom ?

[vphantom]

You no longer enforce a maximum number of concurrent connections?

[vphantom]

Sorry sorry, I was just looking at the diff lines. I guess you're changing your multi-threading strategy if you're going from blocking to non-blocking there. I don't really know what I should be evaluating here.

[c-cube]

Oh it's only the accept loop that uses select, so that it's not blocked forever in case we want to stop the server. In particular, `stop` should now work after 1s even if no new client connects.

[vphantom]

Oh! I forgot that the graceful shutdown was still an issue. (I've been neck-deep in unrelated work for so long…) For sure stopping the accept loop as soon as the signal is received, and not after the next client is accepted, is a much more predictable behavior.