MEDIUM: checks: Implement LDAP check using tcp-check rules

MEDIUM: checks: Implement LDAP check using tcp-check rules

A shared tcp-check ruleset is now created to support LDAP check. This way no
extra memory is used if several backends use a LDAP check.

The following sequance is used :

    tcp-check send-binary "300C020101600702010304008000"

    tcp-check expect rbinary "^30" min-recv 14 \
        on-error "Not LDAPv3 protocol"

    tcp-check expect custom

The last expect rule relies on a custom function to check the LDAP server reply.