Reduce the number of system calls for accept

[chambart]

Use accept4 instead of accept to avoid 2 fcntl system calls

accept4 takes flags to tell to create the file descriptor in non blocking mode. This way we don't need to set it later with Unix.set_nonblock.

Also, since we know that all the calls to accept will be on non blocking sockets we can avoid releasing the runtime lock.

[aantron]

Thanks! I'll merge this shortly.

Just a comment:

Also, since we know that all the calls to accept will be on non blocking sockets we can avoid releasing the runtime lock.

The fd might be blocking, as a user can call Lwt_unix.set_blocking listen_fd true (though I know of no good reason to do so).

However, the current implementation in Lwt seems to assume that Unix.accept on a blocking listening socket will not block anyway, as Lwt will only call Unix.accept if the listening socket becomes readable (i.e., has a connection pending). I don't know if that's correct — perhaps the listening socket can become readable and then Unix.accept still blocks. However, if that's possible, this PR will be about equally as buggy as Lwt currently is here, so I think we can ignore this for now :)