Followup to r1631435: add '--transaction' support to 'svnadmin delrevprop'.

* subversion/svnadmin/svnadmin.c
  (cmd_table): Add 't' to delrevprop, tweak text for setrevprop and delrevprop.
  (subcommand_setrevprop): Tweak error message.
  (subcommand_delrevprop): Sanity check the new option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631446 13f79535-47bb-0310-9956-ffa450edef68