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

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

This was by far the largest block of cfg_parse_listen(), about 470
lines. It is now registered in cfg_kws and parsed by
proxy_parse_option(), making the keyword discoverable via "-dKcfg". The
keyword modifier is taken from cfg_curr_kwm since the keyword parsers
don't receive it.

The whole block of non-standard options was a bit rearranged by
dependencies or behaviors so as to factor some code, mode checks and
flags setting.