symbolic links should be recorded in the registry

[mf2k (Frank Schima)]

Symbolic links created by port should be recorded in the registry. For example, the python_select port creates symlinks to a specific version of python and it's helper programs.

$ ls -l /opt/local/bin/python
lrwxr-xr-x  1 root  admin  24 Jan 23 15:05 /opt/local/bin/python -> /opt/local/bin/python2.7

$ port provides /opt/local/bin/python
/opt/local/bin/python is not provided by a MacPorts port.

$ port contents python_select
Port python_select contains:
  /opt/local/etc/select/python/base
  /opt/local/etc/select/python/none
  /opt/local/etc/select/python/python25-apple
  /opt/local/etc/select/python/python26-apple
  /opt/local/etc/select/python/python27-apple