CLEANUP: cfgparse-listen: move "acl" to its own parser
The keyword is now registered in cfg_kws and parsed by proxy_parse_acl(),
making it discoverable via "-dKcfg". Care was taken to release the
errptr returned by parse_acl().
The code could also have been moved to acl.c but it appeared strange there
because acl.c doesn't really deal with first-level keywords and doesn't
care about proxies, and here we're in a proxy section, so aftre all,
cfgparse-listen seems to remain the best place for the function.