MINOR: cfgparse: Add functions to backup and restore registered sections

MINOR: cfgparse: Add functions to backup and restore registered sections

This feature will be used by the stream processing offload engine (SPOE) to
parse dedicated configuration files without mixing HAProxy sections with SPOE
sections.

So, here we can back up all sections known by HAProxy, unregister all of them
and add new ones, dedicted to the SPOE. Once the SPOE configuration file parsed,
we can roll back all changes by restoring HAProxy sections.