Change 'clean' action mode and buld dirs structure

Change 'clean' action mode: add the possibility to clean only
the compiled objects directories ('mod' and 'obj') or only the
compiled targest. Add two new switches to 'clean' action mode:

- FoBiS.py clean -only_obj    # clean only compiled objects dires
- FoBiS.py clean -only_target # clean only compiled targets
- FoBiS.py clean              # clean all as previous version

Change the build dirs structure: change 'dexe' switch to 'build_dir',
'dmod' to 'mod_dir' and 'dobj' to 'obj_dir'. Move 'obj_dir' and 'mod_dir'
inside 'build_dir': 'obj_dir' and 'mod_dir' become relative paths to
'build_dir'. Change, consequently, the 'fobos' file options:

- 'dexe' change to 'build_dir'
- 'dmod' change to 'mod_dir'
- 'dobj' change to 'obj_dir'