Handle --keep-above passed through to editor_command_line

Handling of nm-connection-editor command line arguments was
changed in f3782da2. Since that commit, most args are handled
by `editor_command_line`, but one (`--keep-above`) is handled
by `main`. Unfortunately, if `--keep-above` appears with other
arguments, `main` does not strip it from argv and parsing in
`editor_command_line` fails with "Failed to parse options".

To handle this, let's just have `editor_command_line` recognize
but ignore `--keep-above` if it encounters it.

GNOME/network-manager-applet#1

Signed-off-by: Adam Williamson <awilliam@redhat.com>
5 jobs for fix-keep-above in 23 minutes and 3 seconds (queued for 9 seconds)
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00:15:59

 
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passed fedora_autotools_full #40660

00:06:12

passed fedora_autotools_minimal #40659

00:04:17

passed fedora_meson_full #40658

00:02:36

passed fedora_meson_minimal #40657

00:04:24