backport.pl: Update for merger of 'svnauth' into 'svn auth'.

* tools/dist/backport.pl
  ($SVNAUTH): Remove.
  ($AVAILID): Track merger.  While here, push some complexity into the child,
    to accommodate large svn.simple directories.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606356 13f79535-47bb-0310-9956-ffa450edef68