parse_boolean: require exact matches

parse_boolean: require exact matches

Require exact matches in all cases instead of treating strings
starting with 't' ('f') as true (false).

This is required for config_parse_protect_system to parse ProtectSystem=full
correctly: it uses parse_boolean and only tries a more specific parsing
function if that did not return a valid result. Thus "full" was treated as
"false" before.