_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.
pluto: Don't attempt to read attributes when there's just padding

Libreswan, unlike cisco, likes to add padding when transform payload attributes
don't line up to 4-octet boundaries while it doesn't seem to be too happy about
padding, being non-interoperable with itself (unless "ikepad" is turned off):

002 "conn" #4: modecfg: Sending IP request (MODECFG_I1)
005 "conn" #4: Received IPv4 address: 10.0.0.10/32
005 "conn" #4: Received IP4 NETMASK 255.255.255.255
005 "conn" #4: Received DNS server 8.8.8.8
005 "conn" #4: Received Domain: yolo
005 "conn" #4: Received Banner: swag
005 "conn" #4: Received subnet 192.168.100.156/32, maskbits 32
003 "conn" #4: not enough room in input packet for ISAKMP ModeCfg attribute (remain=2, sd->size=4)

RFC 2408 3.6 seems to be a bit unclear about how to pad the attributes
referring to "any padding." Let's just assume anything shorter than four octets
(minimal attribute size) at the end is padding and don't attempt to read
through it.