gpg: Add hidden key-edit subcommand "change-usage".
* g10/keyedit.c (cmdCHANGEUSAGE): New. (cmds): Add command "change-usage". (keyedit_menu): Handle that command. (menu_changeusage): New. * g10/keygen.c (keygen_add_key_flags): New. (ask_key_flags): Add optional arg current.
- Signed-off-by: Werner Koch <wk@gnupg.org>