agent: Add command DELETE_KEY.
* agent/command.c (cmd_delete_key): New. * agent/findkey.c (modify_description): Add '%C' feature. (remove_key_file): New. (agent_delete_key): New. * agent/command-ssh.c (search_control_file): Make arg R_DISABLE optional. * configure.ac: Require libgpg-error 1.13.