plugin-manager,plugin: run pre-probing filters early

For each port, we will construct the list of plugins to test with. In that list
we will include those plugins which are likely to handle a given port, so we
will skip all those which aren't.

To see if a plugin is likely or not, we will run the pre-probing filters before
adding them to the list, with the new `mm_plugin_discard_port_early()'. This
method will return one of these hints:

 * UNSUPPORTED: The plugin will not be able to handle this port.
 * MAYBE: The plugin may handle this port.
 * LIKELY: The plugin may (very likely) handle this port.
 * SUPPORTED: If any plugin should support the port, this is it.

Plugins reported to be 'likely' supporting the port will be probed before the
ones reported just as 'maybe'.

If a plugin reports 'supported' only that one and the fallback generic ones will
be tried.

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