Utilize _nm_setting_to_dbus() to serialize the setting. The main reason
is that this way we can also print the more complicated values
g_strdup_value_contents() can't grok, e.g. the GArrays and GHashTables.

Some effort was spent on tidying up the results in a manner it was done
previously, instead of reducing this to a plain g_variant_print(). It
looks good that way:

Before:

  vpn
    service-type : "org.freedesktop.NetworkManager.VPN.Novpn" (s)
    user-name : NULL (sd)
    persistent : FALSE (sd)
    data : ((GHashTable*) 0xc61060) (s)
    secrets : ((GHashTable*) 0xdda640) (s)
    timeout : 0 (sd)

After:

  vpn
    service-type : 'org.freedesktop.NetworkManager.VPN.Novpn'
    data : {'gateway': 'novpn.example.com', 'username': 'hello'}
    secrets : {'password': 'world'}

Note that no effort was spent on printing the defaults. There are
multiple ways that could be achieved, but I'm not sure it would be all
that necessary given this is really just a quick'n'dirty debugging facilty.