gpg: Print indicator for unknown key capability.

* g10/keylist.c (print_capabilities): Print '?' for unknown usage.

doc/DETAILS

@@ -161,6 +161,7 @@ described here.
     - s :: Sign
     - c :: Certify
     - a :: Authentication
+    - ? :: Unknown capability
 
     A key may have any combination of them in any order.  In addition
     to these letters, the primary key has uppercase versions of the

g10/keylist.c

@@ -627,6 +627,9 @@ print_capabilities (PKT_public_key *pk, KBNODE keyblock)
   if ((use & PUBKEY_USAGE_AUTH))
     es_putc ('a', es_stdout);
 
+  if ((use & PUBKEY_USAGE_UNKNOWN))
+    es_putc ('?', es_stdout);
+
   if (keyblock)
     {
       /* Figure out the usable capabilities.  */