Flags should be explicit

[flungo]

Currently, flag options are either true or yes and any other value results in the flag not being set (implying the opposite). In order for them to function properly in the context of precedence, they should be explicit whether true or false. Implementing this will remove the need for negated options (e.g. DOCKER_DISABLE_SIG_PROXY) and will allow warnings when set to any other non-empty value.