MINOR: cli/threads: make "show fd" report thread_sync_io_handler instead of "unknown"

The output was confusing when the sync point's dummy handler was shown.

This patch should be backported to 1.8 to help with troubleshooting.
diff --git a/src/cli.c b/src/cli.c
index ada45a8..f19b5f1 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -817,6 +817,7 @@
 			     (fdt.iocb == conn_fd_handler)  ? "conn_fd_handler" :
 			     (fdt.iocb == dgram_fd_handler) ? "dgram_fd_handler" :
 			     (fdt.iocb == listener_accept)  ? "listener_accept" :
+			     (fdt.iocb == thread_sync_io_handler) ? "thread_sync_io_handler" :
 			     "unknown",
 			     fdt.thread_mask, fdt.update_mask);