MSC: int umask( int permission )

	- prototype in io.h, sys\types.h, sys\stat.h

	- changes permision in which a file may be accessed
	- permission	= S_IWRITE	write is forbidden
			= S_IREAD	read is forbidden  (ignored by DOS)
			= S_IWRITE | S_IREAD  no reading or writing

	- returns previous permission value