Fix to work more like GNU realpath for relative PATHs.

For instance the current behavior that is different with Ohlah realpath:

   Current Ohloh realpath:

      jsantiago@jlsws:~/test99/realpath$ pwd
         /home/jsantiago/test99/realpath

      jsantiago@jlsws:~/test99/realpath$ ./realpath .
         /data/home/jsantiago/test99/realpath/

      jsantiago@jlsws:~/test99/realpath$ ./realpath ../
         /data/home/jsantiago/test99/

      jsantiago@jlsws:~/test99/realpath$ ./realpath ../realpath/.
         /data/home/jsantiago/test99/realpath/

   GNU realpath (Notice there is no / at the end)

      jsantiago@jlsws:~/test99/realpath$ pwd
         /home/jsantiago/test99/realpath

      jsantiago@jlsws:~/test99/realpath$ realpath .
         /data/home/jsantiago/test99/realpath

      jsantiago@jlsws:~/test99/realpath$ realpath ../
         /data/home/jsantiago/test99

      jsantiago@jlsws:~/test99/realpath$ realpath ../realpath/.
         /data/home/jsantiago/test99/realpath

   After my change Ohloh realpath:

      jsantiago@jlsws:~/test99/realpath$ pwd
         /home/jsantiago/test99/realpath

      jsantiago@jlsws:~/test99/realpath$ ./realpath .
         /data/home/jsantiago/test99/realpath

      jsantiago@jlsws:~/test99/realpath$ ./realpath ../
         /data/home/jsantiago/test99

      jsantiago@jlsws:~/test99/realpath$ ./realpath ../realpath/.
         /data/home/jsantiago/test99/realpath

GNU raelpath I am testing against:

   jsantiago@jlsws:~/test99/realpath$ pwd
      /home/jsantiago/test99/realpath

   jsantiago@jlsws:~/test99/realpath$ which realpath
      /usr/bin/realpath

   jsantiago@jlsws:~/test99/realpath$ realpath --version
      realpath (GNU coreutils) 8.15
      Copyright (C) 2012 Free Software Foundation, Inc.
      License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
      This is free software: you are free to change and redistribute it.
      There is NO WARRANTY, to the extent permitted by law.

      Written by Pádraig Brady.