MEDIUM: cfgparse: post section callback

MEDIUM: cfgparse: post section callback

This commit implements a post section callback. This callback will be
used at the end of a section parsing.

Every call to cfg_register_section must be modified to use the new
prototype:

    int cfg_register_section(char *section_name,
                             int (*section_parser)(const char *, int, char **, int),
                             int (*post_section_parser)());