pppd: Ignore received EAP messages when not doing EAP

pppd: Ignore received EAP messages when not doing EAP

This adds some basic checks to the subroutines of eap_input to check
that we have requested or agreed to doing EAP authentication before
doing any processing on the received packet.  The motivation is to
make it harder for a malicious peer to disrupt the operation of pppd
by sending unsolicited EAP packets.  Note that eap_success() already
has a check that the EAP client state is reasonable, and does nothing
(apart from possibly printing a debug message) if not.

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