Update ppp_available and the no_ppp_msg message

These days almost all Linux systems are going to be using the "new"
driver, i.e. the one that's been in the kernel since before 2.4.0.
This updates the message we print if the kernel doesn't seem to
support ppp to be more appropriate for the new driver.

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