CLEANUP: cfgparse-listen: move the backend one-arg keywords to their own parser

CLEANUP: cfgparse-listen: move the backend one-arg keywords to their own parser

"retries", "http-reuse", "http-send-name-header", "dynamic-cookie-key",
"load-server-state-from-file" and "server-state-file-name" all require
the backend capability and take a single argument, so they were moved
together to a new proxy_parse_be_opts() function registered in cfg_kws,
making them discoverable via "-dKcfg". The code was slightly cleaned up
while moved, but nothing sensitive (removed some flag dance which already
caused bugs in the past, etc).