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

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

The patch adds support for 'import/export target none' (or '[]' to
specify an empty set). It can be used when we do not want to import/export
any route from/to the VRF, or if we prefer to set the RT it in filters
(e.g., adding a different RT for different IP prefixes).

The patch also adds support for 'import target all', i.e. all VPN routes
are imported in the VRF IP table regardless of the RTs. Useful when more
complexx policy implemented in filters.

Based on patches from Sébastien Parisot <sparisot@iliad-free.fr>, thanks!