CLEANUP: cfgparse-listen: move "source" "usesrc" and "interface" to their own parser

CLEANUP: cfgparse-listen: move "source" "usesrc" and "interface" to their own parser

The keywords are now registered in cfg_kws and parsed by
proxy_parse_source(), making them discoverable via "-dKcfg". The loop
was slightly reorganized. We cannot really use the BUG_ON() trap here
for unregistered keywords due to the loop that tries to consume as many
sub-keywords as possible (since "usesrc" and "interface" are in fact
options of "source" parsed as if they were first-level ones).