CLENAUP: cfgparse-listen: move the "server" keywords to their own parser

CLENAUP: cfgparse-listen: move the "server" keywords to their own parser

"server", "default-server" and "server-template" only differ by the
flags they pass to parse_server(), so they were moved together to a new
proxy_parse_server() function registered in cfg_kws, making them
discoverable via "-dKcfg". parse_server() emits its own messages, so the
new parser only has to turn a fatal error into a negative return value.