ppp: Move ioctl definitions from if_ppp.h to new ppp-ioctl.h

This moves the definitions of the ioctls, constants and structures
relating to the ppp_generic interface to userspace out from if_ppp.h
to a new file, ppp-ioctl.h.  The new file has my copyright since I
designed and implemented the ppp_generic interface in the late 1990s.
None of the contents of this file comes from the original if_ppp.h
published by Carnegie Mellon University.

Of the remainder of if_ppp.h, only the PPP_MTU definition was being
used, and this replaces the uses of it with PPP_MRU (which is identical).
Therefore, this replaces the entire file with the single line

#include <linux/ppp-ioctl.h>

which clearly doesn't contain any CMU code.  Thus I have removed the
CMU copyright notice with its problematic advertising clause, and in
fact since it's only one trivial line I have not added any other
copyright notice.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>