MINOR: cfgparse-global: move tune options in global keywords list

MINOR: cfgparse-global: move tune options in global keywords list

In order to clean up cfg_parse_global() and to add the support of the new
MODE_DISCOVERY in configuration parsing, let's move the keywords related to
tune options into the global keywords list and let's add for them two dedicated
parsers. Tune options keywords are sorted between two parsers in dependency of
parameters number, which a given tune option needs.

tune options parser is called by section parser and follows the common API, i.e.
it returns -1 on failure, 0 on success and 1 on recoverable error. In case of
recoverable error we've previously returned ERR_ALERT (0x10) and we have emitted
an alert message at startup. Section parser treats all rc > 0 as ERR_WARN. So in
case, if some tune option was set twice in the global section, tune
options parser will return 1 (in order to respect the common API), section
parser will treat this as ERR_WARN and a warning message will be emitted during
process startup instead of alert, as it was before.