[Tile-serving] [openstreetmap/mod_tile] Replace select() by poll(). (#239)

[Tom Hughes]

I don't think I have  a saved strace but it gets stuck in a loop calling `select` and getting an error (`EBADF` I think) back.

Fundamentally bad things are going to happen as soon as a connection has an FD over 1023 though as the code just goes ahead and tries to add that to the select set, which writes beyond the end of the set so trashes some random bit of memory.

Then it calls select with a limit over 1023 but the extra bits probably get lost in libc or something and the kernel winds up erroring.

I'll try and get a trace if it happens again but `select` is fundamentally bad both because of the fixed set size (unless you carefully redefine `FD_SETSIZE` to increase it at compile time) and `poll` generally scales much better. See http://0pointer.net/blog/file-descriptor-limits.html for some recent advice on this area...

Obviously I can reduce NOFILE to 1024 on our servers but that will just cause different issues if we're reaching the point that we need that many descriptors.

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