CLEANUP: cfgparse-listen: add parser for "log", "log-tag" and "unique-id-header"

CLEANUP: cfgparse-listen: add parser for "log", "log-tag" and "unique-id-header"

The three keywords were moved to a new proxy_parse_log_opts() function
registered in cfg_kws, making them discoverable via "-dKcfg". The "log"
keyword consults cfg_curr_kwm to detect "no log", and the header name is
now allocated before the previous one is released. Arguably we could have
moved unique-id-header in its own function, or move them all to another
existing function, this doesn't change much. They were taken together
as they were grouped in the original code block.