Implement 'svnadmin verify -t (--transaction)'.

* subversion/svnadmin/svnadmin.c
  (options_table): Add '-t'.
  (sub_main): Parse '-t'.
  (cmd_table."verify"): Take '-t'.
  (svnadmin_opt_state.txn_id): New struct member.
  (subcommand_verify): Use '-t'.

* subversion/svnlook/svnlook.c
  (main): Make 'svnlook verify' suggest 'svnadmin verify', inspired by
    'svn undo'.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1462353 13f79535-47bb-0310-9956-ffa450edef68