[BUG] log: option tcplog would log to global if no logger was defined

[BUG] log: option tcplog would log to global if no logger was defined

Romuald du Song reported a strange bug causing "option tcplog" to
unexpectedly use global log parameters if no log server was declared.

Eventhough it can be useful in some circumstances, it only hides
configuration bugs and can even cause traffic logs to be sent to
the wrong logger, since global settings are just for the process.

This has been fixed and a warning has been added for configurations
where tcplog or httplog are set without any logger. This fix must
be backported to 1.3.20, but not to 1.3.15.X in order not to risk
any regression on old configurations.