L3VPN: Adding support for import/export target none and import target all

[Sébastien PARISOT]

Hello BIRD team,

This patch (for master branch) contains two commits for L3VPN protocol:

- 0001: It adds support for "import/export target none" (or "[]" to specify an empty set, which is the same). For example it can be used when we do not want to import/export any route from/to the VRF, of if we prefer to set the RT it in filters (adding different RT for different IP prefixes for example).

- 0002: It adds support for "import target all", ie all VPN routes are imported in the VRF IP table regardless of the RTs. Useful when we need to import any route or we want to have a more complex policy implemented in filters.

Thanks!
--
Sébastien
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