pppd: Add client CHAP authentication timeout

If CHAP authentication is required with the peer but this is never
completed (either because the server never sends the challenge or
because the client doesn't receive the outcome) then the client
will wait forever, relying on the server to terminate the connection.

There are options for server side retries but a client side timeout
option is required to prevent the client from getting stuck if the
server won't terminate the connection. This is defaulted to 60 seconds.

Signed-off-by: Simon Arlott <git@sa.me.uk>