Remove more stuff that we don't need to distribute

This removes some files from include/linux that any halfway-modern
distro will have under /usr/include, and removes the stuff from the
linux/ directory that we haven't included in the tarballs for ages.
This also moves some MPPE macros from include/{linux,net}/ppp-comp.h
to a new pppd/mppe.h.

Signed-off-by: Paul Mackerras <paulus@samba.org>