ssh-keygen: Make option parsing work with OpenSSH < 7

OpenSSH 7.x:

usage: ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa | rsa1]
                  [-N new_passphrase] [-C comment] [-f output_keyfile]
       ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f ...]

Earlier:

usage: ssh-keygen [options]
Options:
-A          Generate non-existent host keys for all key types.
-a number   Number of KDF rounds for new key format or ...

completions/ssh-keygen

@@ -40,7 +40,9 @@ _ssh_keygen()
     esac
 
     if [[ $cur == -* ]]; then
-        COMPREPLY=( $( compgen -W '$( _parse_usage "$1" "-?" )' -- "$cur" ) )
+        local opts=$( _parse_usage "$1" "-?" )
+        [[ -z "$opts" ]] && opts=$( _parse_help "$1" "-?" )  # OpenSSH < 7
+        COMPREPLY=( $( compgen -W "$opts" -- "$cur" ) )
     fi
 } &&
 complete -F _ssh_keygen ssh-keygen