Update the GPLv2 licence text to the current version (with the current
address of the Free Software Foundation, and without the Y2k
compatibility issue).

Rename the GPLv2 licence text to COPYING.GPLv2 (to hopefully make it
clearer that the licence does not apply to all files within the source
tree) and rename COPYRIGHTS to COPYING.

Signed-off-by: Michael Brown <mcb30@ipxe.org>