Add %A and %a formats for fs -stat command to print permissions

This patch adds to fs/shell/Stat.java the missing options of %a and %A.

FileStatus already contains the getPermission() method required for returning symbolic permissions. FsPermission contains the method to return the binary short, but nothing to present in standard Octal format.

Most UNIX admins base their work on such standard octal permissions. Hence, this patch also introduces one tiny method to translate the toShort() return into octal.

Build has already passed unit tests and javadoc.