tree-wide: port more code to use send_one_fd() and receive_one_fd()

Also, make it slightly more powerful, by accepting a flags argument, and
make it safe for handling if more than one cmsg attribute happens to be
attached.

src/journal/journal-send.c

@@ -212,11 +212,6 @@ _public_ int sd_journal_sendv(const struct iovec *iov, int n) {
                 .msg_namelen = offsetof(struct sockaddr_un, sun_path) + strlen(sa.sun_path),
         };
         ssize_t k;
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(int))];
-        } control;
-        struct cmsghdr *cmsg;
         bool have_syslog_identifier = false;
         bool seal = true;
 
@@ -335,26 +330,7 @@ _public_ int sd_journal_sendv(const struct iovec *iov, int n) {
                         return r;
         }
 
-        mh.msg_iov = NULL;
-        mh.msg_iovlen = 0;
-
-        zero(control);
-        mh.msg_control = &control;
-        mh.msg_controllen = sizeof(control);
-
-        cmsg = CMSG_FIRSTHDR(&mh);
-        cmsg->cmsg_level = SOL_SOCKET;
-        cmsg->cmsg_type = SCM_RIGHTS;
-        cmsg->cmsg_len = CMSG_LEN(sizeof(int));
-        memcpy(CMSG_DATA(cmsg), &buffer_fd, sizeof(int));
-
-        mh.msg_controllen = cmsg->cmsg_len;
-
-        k = sendmsg(fd, &mh, MSG_NOSIGNAL);
-        if (k < 0)
-                return -errno;
-
-        return 0;
+        return send_one_fd(fd, buffer_fd, 0);
 }
 
 static int fill_iovec_perror_and_send(const char *message, int skip, struct iovec iov[]) {