merge fdl.texi into magit.texi

Previously fdl.texi was not actually included in magit.texi.  Instead of
including it this patch instead copies the license right into magit.text.
This removes clutter from the top directory, and replacing the text when
moving to a newer version of the license isn't really that much work.

By the way such a newer version (1.3) is available; should we switch to it?