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

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

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

The following sequence is used :

    tcp-check connect default linger

    tcp-check expect rstring "^[0-9]{3}[ \r]" min-recv 4 \
        error-status "L7RSP" on-error "%[check.payload(),cut_crlf]"

    tcp-check expect rstring "^2[0-9]{2}[ \r]" min-recv 4 \
        error-status "L7STS" \
        on-error %[check.payload(4,0),ltrim(' '),cut_crlf] \
        status-code "check.payload(0,3)"

    tcp-echeck send "%[var(check.smtp_cmd)]\r\n" log-format

    tcp-check expect rstring "^2[0-9]{2}[- \r]" min-recv 4 \
        error-status "L7STS" \
        on-error %[check.payload(4,0),ltrim(' '),cut_crlf] \
        on-success "%[check.payload(4,0),ltrim(' '),cut_crlf]" \
        status-code "check.payload(0,3)"

The variable check.smtp_cmd is by default the string "HELO localhost" by may be
customized setting <helo> and <domain> parameters on the option smtpchk
line. Note there is a difference with the old smtp check. The server gretting
message is checked before send the HELO/EHLO comand.