accel-cmd: add -P/--password support

pptp: add the ppp-max-mtu option to match l2tp & sstp

ipv6: dhcpv6: fix PD linklocal route for ipoe clients

 peer linklocal address can't be negotiated in ipoe mode unlike ppp,
 so route may lead to nowhere with non-working PD routing as result.
 so, instead of guessing peer link-local address, use dhcpv6 client
 src address as the gateway.
 since dhcpv6 clients are onlink and there's no dhcpv6 relay support
 (yet), dhcpv6 source would be exactly final linklocal address, no
 matter ppp or ipoe is in use.

 fine tune commit abaa43a3

ppp: fix mtu/mru negotiation in unit-preallocate/unit-cache modes

Send NAK only to unicast requests or if requested Server-ID matches one of gw-ip-address

Ipv6 pool fixes

sstp: drop unnecessary ssl reinitialization

sstp support