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

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

The stick rules are forbidden in a defaults section. They were moved
to a new proxy_parse_stick() function registered in cfg_kws, making
them discoverable via "-dKcfg".

The code is mostly the same, with error paths reworked to avoid chains of
free()+return calls in error paths.