MEDIUM: acl/pattern: factor out the src/dst address fetches

MEDIUM: acl/pattern: factor out the src/dst address fetches

Since pattern_process() is able to automatically cast returned types
into expected types, we can safely use the sample functions to fetch
addresses whatever their family. The lowest castable type must be
declared with the keyword so that config checks pass.

Right now this means that src/dst use the same fetch function for ACLs
and patterns. src6/dst6 have been kept so that configs which explicitly
rely on v6 are properly checked.