The FSF's address changed, and we had a mixture of comment styles for
the GPL file header.  (The style with * at the beginning won out with
580 to 141 in existing files.)

Note: I've intentionally left intact other "variations" of the copyright
header, e.g. whether it says "Zebra", "Quagga", "FRR", or nothing.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>