Use std API to create unbound datagram socket

Unlike the nix API the std type has clear ownership semantics and thus
takes care to close the socket again.

Previously we leaked an FD here.

Fixes #141

src/daemon.rs

@@ -4,6 +4,8 @@ use nix::sys::socket;
 use nix::unistd;
 use std::io::{self, IoSlice};
 use std::os::unix::io::RawFd;
+use std::os::unix::net::UnixDatagram;
+use std::os::unix::prelude::AsRawFd;
 use std::{env, fmt, fs, time};
 
 /// Check for systemd presence at runtime.
@@ -98,14 +100,7 @@ pub fn notify_with_fds(
             .with_context(|| format!("invalid Unix socket path address {}", env_sock))?,
     };
 
-    let sock_fd = socket::socket(
-        socket::AddressFamily::Unix,
-        socket::SockType::Datagram,
-        socket::SockFlag::empty(),
-        None,
-    )
-    .context("failed to open Unix datagram socket")?;
-
+    let socket = UnixDatagram::unbound().context("failed to open Unix datagram socket")?;
     let msg = state
         .iter()
         .fold(String::new(), |res, s| res + &format!("{}\n", s))
@@ -120,7 +115,7 @@ pub fn notify_with_fds(
     };
 
     let sent_len = socket::sendmsg(
-        sock_fd,
+        socket.as_raw_fd(),
         &[msg_iov],
         &ancillary,
         socket::MsgFlags::empty(),