BUG/MINOR: tcpcheck: Be able to expect an empty response

BUG/MINOR: tcpcheck: Be able to expect an empty response

It is not possible to successfully match an empty response. However using
regex, it should be possible to reject response with any content. For
instance:

   tcp-check expect !rstring ".+"

It may seem a be strange to do that, but it is possible, it is a valid
config. So it must work. Thanks to this patch, it is now really supported.

This patch may be backported as far as 2.2. But only if someone ask for it.