BUG/MINOR: startup: leave at first post_section_parser which fails

BUG/MINOR: startup: leave at first post_section_parser which fails

Since we are now iterating on post_section_parser() for a same keyword,
we need to exit at the first ERR_ABORT.

The post_section_parser() is called when parsing a new section, but also
at the end of the file to be called for the last section.

The changes in 4de86bb ("MEDIUM: initcall: allow to register mutiple
post_section_parser per section") should have added tests on the
ERR_ABORT value.

Also pcs->post_section_parser() must be called instead of
cs->post_section_parser() because we could have a NULL ptr.

This bug does not affect anything since we don't use
REGISTER_CONFIG_POST_SECTION() yet.