MEDIUM: checks: Add implicit tcp-check connect rule

MEDIUM: checks: Add implicit tcp-check connect rule

After the configuration parsing, when its validity check, an implicit tcp-check
connect rule is added in front of the tcp-check ruleset if the first non-comment
rule is not a connect one. This implicit rule is flagged to use the default
check parameter.

This means now, all tcp-check rulesets begin with a connect and are never
empty. When tcp-check healthchecks are used, all connections are thus handled by
tcpcheck_main() function.