nominate.pl/backport.pl: When invoked via a symlink called 'n' or 'b' respectively, cd to the directory the symlink resides in.

This allows me to run '../18x/n 1515141. foobar' from a trunk working copy.

* tools/dist/backport.pl
  (File::Basename::dirname): Import.
  (backport_usage, nominate_usage): Document.
  (): When $0 is "b" or "n", use the directory of $0 (probably a symlink).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515149 13f79535-47bb-0310-9956-ffa450edef68