Fixes for systemd seqpacket sockets

[wes-fournier]

Addresses #13

This corrects a couple issues from my previous PR:

• Reexport UnixSeqpacketListenerStream for users of systemd_listen_unix_seqpacket
• I misread accept4's manpage :s We do need to explicitly set O_NONBLOCK before accepting connections. I used fcntl since tokio-seqpacket does not give us a set_nonblocking method.

[molguin92]

nit: on mac OS at least, cargo clippy throws "unused import" warnings

Checking ecdysis v1.1.0 (/Users/molguin/workspace/ecdysis)
warning: unused imports: `FromRawFd` and `RawFd`
 --> src/listener.rs:5:14
  |
5 |         fd::{FromRawFd, RawFd},
  |              ^^^^^^^^^  ^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

warning: `ecdysis` (lib) generated 1 warning (run `cargo clippy --fix --lib -p ecdysis` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.78s

[wes-fournier]

fixed imports to use the correct feature flag

[molguin92]

nit: please run cargo sort to make sure Cargo.toml is properly formatted (unrelated to these changes but would be nice 🙏🏽 ).

Left some other comments here and there too, but lgtm otherwise.

[molguin92]

all good, 🚢 it