Tor keeps a buffer amount of file descriptor to avoid running out when trying to do critical operation such as updating crypto keys or saving the state file. See `ULIMIT_BUFFER` for this which is set to 32. For example, if the system running tor is allowed to open `4096` files, only `4064` of them will be used for network connections. The `tor_accept_socket_nonblocking()` call is done in `connection_handle_listener_read()` which creates a new file descriptor from a listener connection but the open file limit is currently _not_ checked before. Because of that, we accept connection well beyond our limit (set in `ConnLimit_`) which is bad because if we ran out, we can't update for instance our ed25519 keys which results in a violent `exit()` :). I propose we put the open file limit just before we call `socket()`, in `tor_open_socket_with_extensions()` and in `tor_accept_socket_with_extensions()`. This way we centralize the limit check and avoid future issues when adding a new open/accept socket call.