LU-4924 - chmod g+s, touch should not clear sgid bit

	$ mkdir 4924
	$ cd 4924
	$ touch f
	$ chmod u=rwx,g=rwxs f
	$ ls -l f | awk -- '{sub(/\\./, "", $1); print $1, $3, $4 }'
	> -rwxrwsr-- root root
	$ touch f
	$ ls -l f | awk -- '{sub(/\\./, "", $1); print $1, $3, $4 }'
	> -rwxrwsr-- root root
	$ cd ..
	$ rm -rf 4924