xl2tpd: add some default configs needed on most of cases

xl2tpd: add some default configs needed on most of cases

mtu 1400, mru 1400 - on ppp devices, usually we need lower mtu, the existing link mabye is already under a lower MTU
require-mschap-v2 - most of the time l2tp is used in conjunction with windows client who will use this kind of auth
lcp-echo-interval 20, lcp-echo-failure 5 - keep alive 20 seconds interval and dead peer detection after 100 seconds
connect-delay 5000 - wait for up to 5 seconds after the connect script finishes for a valid PPP packet from the peer
nodefaultroute - prevent users from creating default routes with pppd
nodefaultip - disables the default behavior when no local IP address is specified
proxyarp - this will have the effect of making the peer appear to other systems to be on the local ethernet

Signed-off-by: Lucian Cristian <lucian.cristian@gmail.com>