shared/conf-parser: emit a nicer warning for something like "======"

Urlich Windl wrote on the mailing list:
> I noticed that a line of "=======" in "[Service]" cases the message " Unknown lvalue '' in section 'Service'".

This now becomes:
/etc/systemd/system/eqeqeqeq.service:3: Missing key name before '=', ignoring line.