journal-remote: make the child pipe non-blocking

We are going to add this child as a source to our event loop so we don't
want to block when reading data from it as this will prevent us from
processing other events. Specifically this will block the signalfds
which means if we are waiting for data from curl we won't handle SIGTERM
or SIGINT until we happen to get more data.

src/journal-remote/journal-remote.c

@@ -127,6 +127,10 @@ static int spawn_child(const char* child, char** argv) {
         if (r < 0)
                 log_warning_errno(errno, "Failed to close write end of pipe: %m");
 
+        r = fd_nonblock(fd[0], true);
+        if (r < 0)
+                log_warning_errno(errno, "Failed to set child pipe to non-blocking: %m");
+
         return fd[0];
 }