bwlimit: apply limits when there's no override

bwlimit: apply limits when there's no override

Before, 'undef' was equivalent to unlimited, but '0' is the
"explicitly unlimited" value, so if the user doesn't request
an override, apply limits as if the user was unprivileged
(otherwise there's no way for privileged users to explicitly
ask to not override the configured limits).

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>