Add a 'delete' subcommand to svnauth which can be used to delete cached
authentication credentials.

* subversion/svnauth/svnauth.c
  (svnauth__longopt_t): Add subcommand_delete.
  (cmd_table): Add 'delete' subcommand.
  (walk_credentials_baton_t): Add 'delete' flag.
  (walk_credentials): Tell caller to delete creds if the delete flag is set,
   and print the kind and realm of each deleted cred.
  (subcommand_list): Initialise the delete flag to FALSE.
  (subcommand_delete): New. Require at least one PATTERN argument, and
   walk credentials to delete them.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1521469 13f79535-47bb-0310-9956-ffa450edef68