Make sure that all connections returned from NMSettings or created via
AddAndActivateConnection have an NMSettingIP4Config and an
NMSettingIP6Config, with non-NULL methods, and get rid of
now-unnecessary checks for those.

Also move the slaves-can't-have-IP-config checks into the
platform-independent code as well. This also gets rid of spurious
"ignoring IP4/IP6 configuration" warnings in ifcfg-rh when reading a
slave ifcfg file.

Partly based on a patch from Pavel.

https://bugzilla.gnome.org/show_bug.cgi?id=708875