COPYING: Update to MIT Expat License rather than MIT X License

COPYING: Update to MIT Expat License rather than MIT X License

MIT has released software under several slightly different licenses,
including the old 'X11 License' or 'MIT License'.  Some code under this
license was in fact included in X.org's Xserver in the past.  However,
X.org now prefers the MIT Expat License as the standard (which,
confusingly, is also referred to as the 'MIT License').  See
http://cgit.freedesktop.org/xorg/xserver/tree/COPYING

When Wayland started, it was Kristian Høgsberg's intent to license it
compatibly with X.org.  "I wanted Wayland to be usable (license-wise)
whereever X was usable."  But, the text of the older X11 License was
taken for Wayland, rather than X11's current standard.  This patch
corrects this by swapping in the intended text.

In practical terms, the most notable change is the dropping of the
no-advertising clause.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>