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

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

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

The "maxidle" and "maxlife" options were essentially re-merged since
they differed by only the struct member receiving the parsed value.
The code was refactored a little bit since it never happened over the
years when keywords were added, but overall nothing fancy.

There's one minor change though, that's imposed by keyword parsers:
previously it was possible to report up to 3 alerts if multiple args
were invalid, but now it's no longer possible since keyword parsers
can only report one error, so only the first one is reported. No big
deal.