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

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

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

One for the following sequence is used :

    ## If no extra params are set
    tcp-check connect default linger
    tcp-check expect custom  ## will test the initial handshake

    ## If the username is defined
    tcp-check connect default linger
    tcp-check send-binary MYSQL_REQ log-format
    tcp-check expect custom  ## will test the initial handshake
    tcp-check expect custom  ## will test the reply to the client message

The log-format hexa string MYSQL_REQ depends on 2 preset variables, the packet
header containing the packet length and the sequence ID (check.header) and the
username (check.username). If is also different if the "post-41" option is set
or not. Expect rules relies on custom functions to check MySQL server packets.