MINOR: ssl/cli: show certificate status in 'show ssl cert'

Display the status of the certificate in 'show ssl cert'.

Example:

  Status: Empty
  Status: Unused
  Status: Used
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 83bbaca..a92f9c9 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -11254,6 +11254,14 @@
 			chunk_appendf(out, "*");
 		chunk_appendf(out, "%s\n", ckchs->path);
 
+		chunk_appendf(out, "Status: ");
+		if (ckchs->ckch->cert == NULL)
+			chunk_appendf(out, "Empty\n");
+		else if (LIST_ISEMPTY(&ckchs->ckch_inst))
+			chunk_appendf(out, "Unused\n");
+		else
+			chunk_appendf(out, "Used\n");
+
 		if (ckchs->ckch->cert == NULL)
 			goto end;