backport.pl: Use 'svnauth list' to determine the username, if possible.

* tools/dist/backport.pl
  ($SVNAUTH): New global.
  ($AVAILID): Set from `svnauth list` output, if svnauth is available.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507663 13f79535-47bb-0310-9956-ffa450edef68