wrmdep: Added "update" option

Searches all the "src" and "application" directories of the project for
broken symbolic links for source code files and then remove all .dep
files that relate to the files that no longer exist.  Must be executed
in main project source code folder: $WM_PROJECT_DIR

Patch provided by Bruno Santos

Resolves feature-request http://www.openfoam.org/mantisbt/view.php?id=1941