http://schillix.sourceforge.net/opensolarisman/man1/chmod.1.html
For directories, the setgid bit cannot be set (or cleared)
in absolute mode; it must be set (or cleared) in symbolic
mode using g+s (or g-s).
t.j. neresetuj tie bity cislami, ale parametrom g-s, a pre ten druhy bit si najdi v pokece ze jaky parameter.