oconfig: Allow empty statement lists (in blocks and files).

oconfig: Allow empty statement lists (in blocks and files).

This allows to use empty blocks (which is useful during development and
testing) and empty files (which may happen when including config directories,
cf. Debian #592881).

In order not to generate a shift/reduce error, rather than allowing a
'statement_list' to be empty, this has been implemented by explicitly allowing
empty blocks and an empty 'entire_file'.