systemd: replace sendmsg with sendto shortcut

There is a problem with calculating .msg_namelen field
of msghdr struct. Instead of

    .msg_name   = &sa,
    .msg_namelen = sizeof(sa.sun_family) + strlen(sd_unix_path),

it must set as

    .msg_namelen = sizeof(sa) // larger value than current invalid one

It works on linux but when I tried to enable this feature for macOS
it didn't (maybe because of different order of fields in the struct).

Instead of fixing calculation, I've replaced original sendmsg call
with sendto, because it's a convenient shortcut which
simplifies code and can prevent such mistakes.

Required for #4436

(cherry picked from commit 89aae30c)

src/systemd.c

@@ -100,23 +100,14 @@ int systemd_notify(const char *message) {
 	struct sockaddr_un sa = {
 		.sun_family = AF_UNIX,
 	};
-	struct iovec vec = {
-		.iov_base = (char  *)message,
-		.iov_len  = (size_t )strlen(message)
-	};
-	struct msghdr msg = {
-		.msg_iov    = &vec,
-		.msg_iovlen = 1,
-		.msg_name   = &sa,
-	};
 
-	msg.msg_namelen = sizeof(sa.sun_family) + strlen(sd_unix_path);
 	strncpy(sa.sun_path, sd_unix_path, sizeof(sa.sun_path));
 	if (sa.sun_path[0] == '@')
 		sa.sun_path[0] = '\0';
 
 	say_debug("systemd: sending message '%s'", message);
-	ssize_t sent = sendmsg(systemd_fd, &msg, MSG_NOSIGNAL);
+	ssize_t sent = sendto(systemd_fd, message, (size_t) strlen(message),
+		MSG_NOSIGNAL, (struct sockaddr *) &sa, sizeof(sa));
 	if (sent == -1) {
 		say_syserror("systemd: failed to send message");
 		return -1;