Add an egs++ egsExpandPath global function to expand the FIRST
environment variable found anywhere in its string argument:
  - Expands Linux or Windows style environment variable
  - Expands the environment variable
  - Appends folder separator if missing
  - Replaces backslashes with slashes
  - Removes duplicate slashes in the expanded path

This function assumes that there is only one environment variable in the
absolute file path, and that it is a directory.