MINOR: cfgparse: implement experimental config keywords

MINOR: cfgparse: implement experimental config keywords

Add a new flag to mark a keyword as experimental. An experimental
keyword cannot be used if the global 'expose-experimental-directives' is
not present first.

Only keywords parsed through a standard cfg_keywords lists in
global/proxies section will be automatically detected if declared
experimental. To support a keyword outside of these lists,
check_kw_experimental must be called manually during its parsing.

If an experimental keyword is present in the config, the tainted flag is
updated.

For the moment, no keyword is marked as experimental.