backport.pl interactive mode: Fix a bug when $AVAILID ended up being undef.

* tools/dist/backport.pl
  ($AVAILID): When failing to initialize this, make the warning more prominent
    and start requesting permission to continue.
  (nominate_usage): Tolerate $AVAILID being undef.
  (warned_cannot_commit): New helper.
  (vote, nominate_main, handle_entry): Verify that $AVAILID is defined where
    needed.  When it is undefined, warn or bail.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606354 13f79535-47bb-0310-9956-ffa450edef68