nominate.pl: New script.

This actually reuses some of backport.pl's internal helper function, so I made
it an alias to backport.pl that switches on argv[0].  This works for now; using
a proper library (stdlib or otherwise) may be done in the future.

* tools/dist/nominate.pl: New symlink.
* tools/dist/backport.pl
  (main, usage): Rename to..
  (backport_main, backport_usage): .. these, respectively.

* tools/dist/backport.pl
  (Text::Wrap::wrap, Tie::File): Import.
  (nominate_usage, nominate_main): New subs.
  (): Dispatch to backport_main() or nominate_main() based on $0.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511627 13f79535-47bb-0310-9956-ffa450edef68