bug#23853: chmod can unset sticky bits using octet format

[westlake]

According to chmod's manpage,
"chmod preserves a directory's set-user-ID and set-group-ID bits unless 
you explicitly specify otherwise.  You can set or clear the bits with 
symbolic modes like u+s and g-s, and you can set (but not clear) the 
bits with a numeric mode."

contrary to documentation, chmod can otherwise clear sticky bits using 
the octet notation but by using a 5th octal

commands used,
mkdir abc
chmod u+s,g+s abc
chmod 0755 abc (no effect on the sticky bits)
chmod 00755 abc (sticky bits cleared using the 5th octal position)