MEDIUM: proxy/checks: Register a keyword to parse tcp-check rules

MEDIUM: proxy/checks: Register a keyword to parse tcp-check rules

The keyword 'tcp-check' is now parsed in a dedicated callback function. Thus the
code to parse these rules is now located in checks.c. In addition, a deinit
function have been added to release proxy tcp-check rules, on error or when
HAProxy is stopped.

This patch is based on Gaetan Rivet work. It uses a callback function registerd
on the 'tcp-check' keyword instead, but the spirit is the same.