Read(.backup.free_space, "/tmp")
	(3752236)
	Read(.backup.free_space, "/abcd1")
	(-1) // Directory does not exist or is not a directory
	Read(.backup.free_space, "/abcd2")
	(-1) // Cannot find '/bin/df' binary
 Agent calls command '/bin/df $directory' and returns the available space
 in 1K blocks. -1 is returned in case of any error, such as 'Directory does
 not exist', 'Object is not a directory', '/bin/df does not exist on the
 system' or 'Directory not defined'. Every error should appear in the y2log.