ifupdown2 does not exactly respect ifupdown1 interfaces selection.
It is a bit messed up between auto/all/allow/IFLIST.

In ifupdown1, you must provide a -a/--all or IFLIST (except for query -l command).
The --allow CLASSes are used for filtering interfaces to modify.
The -a/--all command set an 'auto' CLASS if no allowed CLASS are provided.

`ifup -a` command, UP all interfaces with the auto CLASS.
`ifup eth1` command, UP eth1 interface (no CLASS filtering).
`ifup -a --allow special`, only UP interfaces with the special CLASS.
`ifup --allow hotplug eth4`, only UP eth4 interface if it has the hotplug CLASS.

For query command, the -a/--all is implicit.

In ifupdown2, the --allow CLASS is used as a way to retrieve interfaces.
ifup/ifdown neither `--allow hotplug eth4` nor `-a --allow special` work.
ifquery command returns duplicate interfaces in output.

This variable is passed to multiple functions where interfaces are
expanded if a specific pattern is given.

Sometimes, functions forget to do the expand and fail to give a result...
Remaining expand in the code can be removed later without impact.

Use the new methods to create a standard interface selection for
ifup, ifdown, ifquery and ifreload operations.

ifquery and ifreload have a tricky implicit configuration, but I have
tried to not modify the ifupdown2 behavior in any way.

There is one changement on the -a/--all option that can be present or not
when implicit. Now, the `ifreload -a -c` command works for example.

Manage the IFLIST or -a/--all input for selection.
Early call of _preprocess_ifacenames to control interfaces.
Use --allow CLASS as a filter and not as an interfaces selector.

* The 'no ifaces found matching' exception has been replaced by a quiet early return

* The 'no ifaces found matching' exception has been replaced by a quiet early return

* The 'no ifaces found matching' exception has been replaced by a quiet early return

Reload operation uses a -a/--all option without --allow auto as default.
This command has an impact on ALL interfaces by default.

The --allow can be set to filter the targetted interfaces.

I do not really understand why the IFLIST is disable on this operation.
It could be very useful.

ifupdown1 and ifupdown2 man are missleading on the -a/--all option.

For ifupdown compatibility, '--all' implies '--allow auto'.
'--allow-all' parameter offers a way to have all interfaces without implicit filter.

On ifquery, --all is implicit when no interfaces are provided. So 'ifquery --allow-all'
shows information on all interfaces.