pluto: Split modecfg attribute addition into a separate function

This will make possible to run the code path outside the loop that adds the
requested attributes (and make it possible to add unrequested ones).
pluto: Always send modecfg banner and domain if they are set

According to the documentation they ought to be sent when the operator
configured them. Likewise, the split tunelling information should be sent
if the network is configured.

We loose track of whether they have been requested anyway thus we wouldn't
send them at all. Certain clients require them though (libreswan won't
invoke the NetworkManager helper if DNS info and DOMAIN is not set.)
_updown.*: Fix NetworkManager callback

Letting NM know is not just specific to resolver configuration management, NM
always needs to know if setting up the connection succeeded or timed out.

Moreover, it's probably not a good idea to unset the variables upon disconnect
as it would be nice if NetworkManager could identify the connection that goes
down.