editor: support IPv6 'disabled' method

NetworkManager 1.20 introduced the 'disabled' IPv6 method. Support it.

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8 jobs for master in 18 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #430855
fedora28_dist

00:06:17

 
  Test
passed #430860
centos7

00:06:54

passed #430859
fedora_autotools_full

00:06:31

passed #430858
fedora_autotools_minimal

00:04:17

passed #430857
fedora_meson_full

00:04:13

passed #430856
fedora_meson_minimal

00:05:48

 
  Deploy
passed #430861
pages

00:05:08

passed pages:deploy

00:00:00