net: ppp: ipcp: negotiate DNS servers and optionally use them

We now negotiate DNS servers in the IPCP configuration. This has been
observed to speed up the connection setup. The received DNS servers
are used by the DNS resolver library, but we leave it optional since
the static server list might be preferable.

Increase MAX_IPCP_OPTIONS to 4 so that we can nack all RFC 1877
options.

Signed-off-by: Göran Weinholt <goran.weinholt@endian.se>