Make move error condition handling a bit more secure by hiding
an api and making a few checks occur earlier.

* subversion/include/private/svn_wc_private.h
  (svn_wc__delete_internal): Remove function. Users can just use
    svn_wc_delete4 or svn_wc__move2().

* subversion/libsvn_wc/copy.c
  (svn_wc__move2): Call svn_wc__db_op_delete() directly.

* subversion/libsvn_wc/delete.c
  (copy_or_move): Check for some cases of invalid moves.
  (svn_wc__delete_internal): Rename to...
  (svn_wc_delete4): ... this.
  (svn_wc_delete4): Remove wrapper function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1451258 13f79535-47bb-0310-9956-ffa450edef68