Add support for disabling connectivity checking via D-Bus API

[James Henstridge <james jamesh id au>]

We're looking at enabling NetworkManager's connectivity checking in
the next Ubuntu release, and one of the concerns has been privacy
issues of regularly pinging a particular server while users are
online.  One option is to include the connectivity check configuration
in a separate package that the user can uninstall, but that isn't
particularly discoverable.

I had a go at implementing an alternative approach, that I attached to
this bug report:

https://bugzilla.gnome.org/show_bug.cgi?id=785117

This adds two D-Bus properties:

ConnectivityCheckAvailable: true if a connectivity URI has been
configured (read-only)
ConnectivityCheckEnabled: true if connectivity checking is enabled (read/write)

The second is implemented by setting the connectivity check interval
to 0 in NetworkManager-intern.conf.

The idea is that a privacy control panel could optionally show an
extra toggle if ConnectivityCheckAvailable is true, and have that
toggle control ConnectivityCheckEnabled.  The change will be
persistent, and doesn't require installing/removing files under
/etc/NetworkManager.

Does this look like a sensible way to implement this feature?  If so,
I can move on to add support for it in gnome-control-center.

James.