svnauthz: Add --is option to accessof subcommand.

* tools/server-side/svnauthz.c
  (svn_authz__cmdline_options_t, options_table, svnauthz_opt_state): Add --is
    option.
  (cmd_table): Accept --is option on accessof and change return documention of
    accessof to reflect that --is changes the return codes.
  (subcommand_accessof): Explicitly initialize booleans and add booleans for
    is states, handle validate of --is argument, implement checking --is
    argument against the permissions granted.
  (sub_main): Add --is option, treat all SVN_ERR_AUTHZ_* errors as validation
    errors.
  


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428563 13f79535-47bb-0310-9956-ffa450edef68