This patch removes the warnings that the check program came up with in
the PPP code: ppp_async.c, ppp_deflate.c and ppp_generic.c.  This
involved adding __user and converting K&R-style function definitions
to ANSI-style.  I also took the time to add some extra comments to
ppp_deflate.c explaining in more detail what each function does and
what its arguments are.