CLEANUP: cfgparse-listen: move "capture" to its own parser

CLEANUP: cfgparse-listen: move "capture" to its own parser

The keyword is now registered in cfg_kws and parsed by
proxy_parse_capture(), making it discoverable via "-dKcfg".

Note that the code was rearranged a bit so that the checks for defaults
section that were common between the 3 variants are now made only once
at the beginning of the function. A little bit more factoring was done
but overall nothing fancy.