Fix/improve comments for LISTEN_BACKLOG_SIZE

[opara-tor]

The backlog size logic that was copied from the standard library was for unix sockets, not inet sockets. Mio's backlog sizes differ greatly from the standard library for inet sockets (128 or smaller). The comments here were confusing/misleading, so I've tried to clarify them.

[Thomasdezeeuw]

Reading https://github.com/rust-lang/rust/blob/4f808ba6bf9f1c8dde30d009e73386d984491587/library/std/src/sys/net/connection/socket.rs#L538-L541 now, these values might be wrong. Might have to revert #1896 or split TCP from Unix values.

[Thomasdezeeuw]

Location in the standard lib:

• Unix: https://github.com/rust-lang/rust/blob/5dbf4069dc98bbbca98dd600a65f50c258fbfd56/library/std/src/os/unix/net/listener.rs#L72
• TCP: https://github.com/rust-lang/rust/blob/5dbf4069dc98bbbca98dd600a65f50c258fbfd56/library/std/src/sys/net/connection/socket/mod.rs#L563-L575

[opara-tor]

I think having separate TCP and unix backlogs would be best. For TCP, I think sticking with the old value of 1024 makes sense to avoid breaking existing users, but maybe the special cases for horizon and haiku could be added.

Should I close this and open an issue instead?