BUG/MINOR: cfgparse: Fix transition between 2 sections with the same name

BUG/MINOR: cfgparse: Fix transition between 2 sections with the same name

When a section's parser is registered, it can also define a post section
callback, called at the end of the section parsing. But when 2 sections with the
same name followed each other, the transition between them was missed. This
induced 2 bugs. First, the call to the post section callback was skipped. Then,
the parsing of the second section was mixed with the first one.

This patch must be backported in 1.8.