varlink: add server write states to disconnect check

While a server is in the VARLINK_PENDING_METHOD or VARLINK_PENDING_METHOD_MORE
states and its write end is disconnected and it gets a POLLHUP, we
should disconnect since it can't write anymore.

In the case of systemd-oomd disconnecting while pid1 was pending-more, this
condition left pid1 in a state where it started throttling from
continually getting POLLHUP.