CLEANUP: cfgparse-listen: simplify the second half of cfg_parse_listen()

CLEANUP: cfgparse-listen: simplify the second half of cfg_parse_listen()

All the proxy keywords are now registered in cfg_kws, so the function
only has to create the sections ("listen", "frontend", "backend" and
"defaults") and to look the keyword up in the registered lists. The
historical indented block that was dealing with known keywords is no
longer necessary and it's the right moment to reindent the few tens
of remaining lines.